skia-bindings downloads ~19 MB from github.com in its build script on
every cold compile. A cold target/ made the rust job fetch it three
times, and one DNS drop turned the fetch into a from-source Skia build
that dies on the missing ninja.
The rust-ci and arm64 cross images bake the pinned archives, checked by
SHA-256, and the four jobs that compile Skia read them through a file://
SKIA_BINARIES_URL. A job on the previous image fetches the pins once.
The script stops before cargo when Cargo.lock leaves the pinned
skia-bindings version, instead of letting the build fall back to source.
enricobuehler
bdb6072443