Check for uncommitted unignored generated archives on CI

This adds a step to `test-fast` to run `git diff` and fail if there are any changes to any tracked files, and sets the newly recognized `GIX_TEST_CREATE_ARCHIVES_EVEN_ON_CI` environment variable for the `nextest` step in which fixture scripts are run. Any changes would indicate a problem that should be addressed, but the main goal here is to catch when a regenerated archive has not been committed. Because adding archives to `.gitignore` would also cause this to pass but is far less often the appropriate solution, this change includes a comment about what the best solution *usually* is. This is added to the `test-fast` jobs but not to `test` job because the `test` job uses `GIT_TEST_IGNORE_ARCHIVES` to prevent already existing archives from being used, which will nearly always result in at least slightly different archives being generated. So in the `test` job this would, in practice, always give a false-positive failure. Because archive generation is (at least currently) suppressed on Windows, this step should rarely if ever fail on Windows even when the problem it is looking for is present. But if it does fail on Windows, then there is a problem that ought to be investigated, and if the problem it is looking for is present, then it will still fail on the other systems, unless the problem is specific to Windows. Running this step on both Ubuntu and macOS (and Windows), rather than just Ubuntu (and Windows), has the benefit of catching some kinds of skew. It is also somewhat simpler than setting it only for some operating systems.

EKEliah Kagan committed 2 years ago