breaking changes to zig build API and improved caching

[andrewrk]

• in Zig build scripts, getOutputPath() is no longer a valid function
  to call, unless setOutputDir() was used, or within a custom make()
  function. Instead there is more convenient API to use which takes
  advantage of the caching system. Search this commit diff for
  exe.run() for an example.
• Zig build by default enables caching. All build artifacts will go
  into zig-cache. If you want to access build artifacts in a convenient
  location, it is recommended to add an install step. Otherwise
  you can use the run() API mentioned above to execute programs
  directly from their location in the cache. Closes zig build system: cache build artifacts and detect cache invalidation #330.
  addSystemCommand is available for programs not built with Zig
  build.
• Please note that Zig does no cache evicting yet. You may have to
  manually delete zig-cache directories periodically to keep disk
  usage down. It's planned for this to be a simple Least Recently
  Used eviction system eventually.
• --output, --output-lib, and --output-h are removed. Instead,
  use --output-dir which defaults to the current working directory.
  Or take advantage of --cache on, which will print the main output
  path to stdout, and the other artifacts will be in the same directory
  with predictable file names. --disable-gen-h is available when
  one wants to prevent .h file generation.
• @cImport is always independently cached now. Closes independently cache @cImports and C object files #2015.
  It always writes the generated Zig code to disk which makes debug
  info and compile errors better. No more "TODO: remember C source
  location to display here"
• Fix .d file parsing. (Fixes the MacOS CI failure)
• Zig no longer creates "temporary files" other than inside a
  zig-cache directory.

This breaks the CLI API that Godbolt uses. The suggested new invocation
can be found in this commit diff, in the changes to test/cli.zig.