ci: fix rusty_v8 binary download unavailable (#7898)

A recent change in rustc or cargo made it so that rusty_v8's `build.rs`,
which is responsible for downloading `librusty_v8.a`, does not get
rebuilt or re-run when its build output directory is restored from the
Github Actions cache.

However, rusty_v8's custom build script does not save the download to
its build output directory; it puts the file in
`target/debug|release/gn_out/obj` instead.

To get CI going again we opted to add `target/*/gn_out` to the Github
Actions cache.

A more robust fix would be make rusty_v8 save the download to the
cargo-designated output directory.

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