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.