WIP: fixes needed by cerbero regarding the windows images

Nirbheek Chauhan requested to merge nirbheek/gstreamer:windows-image-fixes into main

Windows CI image generation happens here, so these changes need to be here.

ci/windows: Remove cargo-vendor sources from the cerbero source cache

Copying the source cache to a running Cerbero CI job on Windows causes
strange / corrupted errors that talk about hard links like:

++ cp -a C:/cerbero/cerbero-sources .
cp: will not create hard link `./cerbero-sources/gstreamer-1.0/subprojects/gst-plugins-good/sys/osxaudio' to directory `./cerbero-sources/cargo-c-0.9.12/cargo-vendor/adler/benches'

So, delete it. The vendored sources are cached by cargo in its own
cache inside CARGO_HOME, which is build/rust/cargo/ and is preserved
in the image. We won't need to redownload any of this.
WIP: ci: Override WINDOWS_IMAGE variable in cerbero CI if needed

Once Cerbero starts using the new .gitlab-image-tags.yml file to
automatically update the windows image it's using, we need to make
sure that Cerbero CI actually tests that image, otherwise we will
inadvertently break it.

See: https://gitlab.freedesktop.org/gstreamer/cerbero/-/merge_requests/938

WIP because the image-changed check isn't implemented yet. Just
opening an MR now so I can link to it from the cerbero MR.

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