Improve Rust workspace build performance

[iliana]

Our Rust builds take too long, primarily because we're doing a separate cargo install for every single binary.

What we're going to try:

• Use cargo build --release instead of cargo install
• Compile all the bins in the workspace at once

Things we need to do the above:

• Use nightly cargo inside the buildroot so we can use --out-dir :(

What we tried:

• sccache (didn't cache much, and didn't help much on top of buildkit's caching)
• lld for linking (saved less than a second)

[iliana]

Crossing out nightly Cargo for now, since I think we can continue to install individual binaries into %{_cross_bindir}. (--bins doesn't let us use --exclude.)