ci: ostree_rs_ext reinstalls Rust and rebuilds dependencies every run

Problem

The ostree_rs_ext GHA job installs rustup and compiles ostree-rs-ext from scratch on every run.

Review context

Sandbox GHA migration review (mtrmac, PR #1 discussion), marked "not now":

(If we preserve the ostree_rs_ext tests,) perhaps we can avoid installing Rust + dependencies on every run, by building into CI images or caching

Related

• Epic #2919
• #2917 — ostree_rs_ext job uses archived ostree-rs-ext repository
• ostree_rs_ext job in .github/workflows/ci.yml