Fix skeleton URLs for CentOS 6 (EOL) and various CI fixes

[mbargull]

This updates the RPM skeleton URLs for CentOS 6 to use http://vault.centos.org (the previous ones aren't valid anymore due to CentOS 6(.10)'s EOL).

Also has some small test/CI fixes.

[mbargull]

This is ready.

There are still some sporadic failures on macOS and Windows:

______________________ test_env_creation_fail_exits_build ______________________
[gw0] darwin -- Python 3.8.5 /Users/runner/work/1/a/miniconda/bin/python
Traceback (most recent call last):
  File "/Users/runner/work/1/s/tests/test_api_build.py", line 1148, in test_env_creation_fail_exits_build
    api.build(recipe, config=testing_config)
  File "/Users/runner/work/1/s/conda_build/api.py", line 186, in build
    return build_tree(
  File "/Users/runner/work/1/s/conda_build/build.py", line 3044, in build_tree
    test(pkg, config=metadata.config.copy(), stats=stats)
  File "/Users/runner/work/1/s/conda_build/build.py", line 2726, in test
    metadata, hash_input = construct_metadata_for_test(recipedir_or_package_or_metadata,
  File "/Users/runner/work/1/s/conda_build/build.py", line 2529, in construct_metadata_for_test
    m, hash_input = _construct_metadata_for_test_from_package(recipedir_or_package, config)
  File "/Users/runner/work/1/s/conda_build/build.py", line 2407, in _construct_metadata_for_test_from_package
    recipe_dir, need_cleanup = utils.get_recipe_abspath(package)
  File "/Users/runner/work/1/s/conda_build/utils.py", line 458, in get_recipe_abspath
    conda_package_handling.api.extract(recipe, recipe_dir)
  File "/Users/runner/work/1/a/miniconda/lib/python3.8/site-packages/conda_package_handling/api.py", line 58, in extract
    SUPPORTED_EXTENSIONS[ext].extract(fn, dest_dir, components=components)
  File "/Users/runner/work/1/a/miniconda/lib/python3.8/site-packages/conda_package_handling/tarball.py", line 135, in extract
    _tar_xf(fn, dest_dir)
  File "/Users/runner/work/1/a/miniconda/lib/python3.8/site-packages/conda_package_handling/tarball.py", line 88, in _tar_xf
    archive_utils.extract_file(tarball)
  File "/Users/runner/work/1/a/miniconda/lib/python3.8/site-packages/conda_package_handling/archive_utils.py", line 15, in extract_file
    raise InvalidArchiveError(tarball, error_str.decode('utf-8'))
conda_package_handling.exceptions.InvalidArchiveError: Error with archive /private/tmp/cb/popen-gw0/test_env_creation_fail_exits_b0/osx-64/_post-link-fails-1.0-0.tar.bz2.  You probably need to delete and re-download or re-create this file.  Message from libarchive was:

Truncated tar archive

________________________ test_build_msvc_compiler[9.0] ________________________
[gw3] win32 -- Python 3.8.5 C:\Users\VssAdministrator\Miniconda3\python.exe
Traceback (most recent call last):
  File "D:\a\1\s\tests\test_api_build.py", line 343, in test_build_msvc_compiler
    api.build(os.path.join(metadata_dir, '_build_msvc_compiler'), python="2.7")
  File "D:\a\1\s\conda_build\api.py", line 186, in build
    return build_tree(
  File "D:\a\1\s\conda_build\build.py", line 3044, in build_tree
    test(pkg, config=metadata.config.copy(), stats=stats)
  File "D:\a\1\s\conda_build\build.py", line 2834, in test
    environ.create_env(metadata.config.test_prefix, actions, config=metadata.config,
  File "D:\a\1\s\conda_build\environ.py", line 913, in create_env
    execute_actions(actions, index)
  File "C:\Users\VssAdministrator\Miniconda3\lib\site-packages\conda\common\io.py", line 88, in decorated
    return f(*args, **kwds)
  File "C:\Users\VssAdministrator\Miniconda3\lib\site-packages\conda\plan.py", line 321, in execute_actions
    execute_instructions(plan, index, verbose)
  File "C:\Users\VssAdministrator\Miniconda3\lib\site-packages\conda\plan.py", line 533, in execute_instructions
    cmd(state, arg)
  File "C:\Users\VssAdministrator\Miniconda3\lib\site-packages\conda\instructions.py", line 67, in PROGRESSIVEFETCHEXTRACT_CMD
    progressive_fetch_extract.execute()
  File "C:\Users\VssAdministrator\Miniconda3\lib\site-packages\conda\core\package_cache_data.py", line 655, in execute
    raise CondaMultiError(exceptions)
conda.CondaMultiError: C:/Users/VssAdministrator/Miniconda3/conda-bld/win-64/conda-build-test-build-msvc-compiler-1.0-0.tar.bz2

_________________________ test_overlinking_detection __________________________
[gw0] win32 -- Python 3.8.5 C:\Users\VssAdministrator\Miniconda3\python.exe
Traceback (most recent call last):
  File "D:\a\1\s\tests\test_api_build.py", line 1435, in test_overlinking_detection
    api.build(recipe, config=testing_config)
  File "C:\Users\VssAdministrator\Miniconda3\lib\site-packages\_pytest\python_api.py", line 728, in __exit__
    fail(self.message)
  File "C:\Users\VssAdministrator\Miniconda3\lib\site-packages\_pytest\outcomes.py", line 153, in fail
    raise Failed(msg=msg, pytrace=pytrace)
Failed: DID NOT RAISE <class 'conda_build.exceptions.OverLinkingError'>

[mingwandroid]

Fantastic, thank you so much @mbargull. Please rebase away!

[github-actions]

Hi there, thank you for your contribution!

This pull request has been automatically locked because it has not had recent activity after being closed.

Please open a new issue or pull request if needed.

Thanks!