ci: Add runner name to rust-cache key

[ahomescu]

This fixes errors caused by reusing Ubuntu 24.04 binaries on the older 22.04 image with older libraries like glibc:

   Compiling z3-sys v0.8.1
error: failed to run custom build command for `z3-sys v0.8.1`

Caused by:
  process didn't exit successfully: `/home/runner/work/c2rust/c2rust/target/release/build/z3-sys-29b81b2e035745ff/build-script-build` (exit status: 1)
  --- stderr
  /home/runner/work/c2rust/c2rust/target/release/build/z3-sys-29b81b2e035745ff/build-script-build: /lib/x86_64-linux-gnu/libm.so.6: version `GLIBC_2.38' not found (required by /home/runner/work/c2rust/c2rust/target/release/build/z3-sys-29b81b2e035745ff/build-script-build)

[kkysen]

Ah, maybe this is why this happened occasionally. I just deleted the cache and it was fixing it temporarily I guess.

[fw-immunant]

Makes sense, thanks for tracking this down.