env: prevent warning from being emitted on venv creation (#420)

* env: prevent warning from being emitted on venv creation

After python/cpython@6811fda
the venv module produces spurious warnings for venv paths which contain
DOS-encoded parts e.g. "USER~1" in "C:\Users\USER~1".
`tempfile.gettempdir()` returns legacy paths like these for
user temp dirs.

MRE:

    python -c "import tempfile
    import venv

    venv.create(tempfile.mkdtemp())"

    Actual environment location may have moved due to redirects, links or junctions.
    Requested location: "C:\Users\RUNNER~1\AppData\Local\Temp\tmpfoobar\Scripts\python.exe"
    Actual location:    "C:\Users\runneradmin\AppData\Local\Temp\tmpfoobar\Scripts\python.exe"

Closes #413.

* tests: fix logging line nos

* fixup! env: prevent warning from being emitted on venv creation

* tests: fix line nos, again

src/build/env.py

@@ -92,7 +92,12 @@ def __enter__(self) -> IsolatedEnv:
 
         :return: The isolated build environment
         """
-        self._path = tempfile.mkdtemp(prefix='build-env-')
+        # Call ``realpath`` to prevent spurious warning from being emitted
+        # that the venv location has changed on Windows. The username is
+        # DOS-encoded in the output of tempfile - the location is the same
+        # but the representation of it is different, which confuses venv.
+        # Ref: https://bugs.python.org/issue46171
+        self._path = os.path.realpath(tempfile.mkdtemp(prefix='build-env-'))
         try:
             # use virtualenv when available (as it's faster than venv)
             if _should_use_virtualenv():

tests/test_env.py

@@ -112,7 +112,7 @@ def test_isolated_env_log(mocker, caplog, package_test_flit):
         ('INFO', 'Installing packages in isolated environment... (something)'),
     ]
     if sys.version_info >= (3, 8):  # stacklevel
-        assert [(record.lineno) for record in caplog.records] == [105, 102, 193]
+        assert [(record.lineno) for record in caplog.records] == [105, 107, 198]
 
 
 @pytest.mark.isolated