ci: Block PRs on Nix build failures (#33688)

Closes #17458

For now we're being conservative and only running CI on changes to the
following files:
- `flake.{nix,lock}`
- `Cargo.{lock,toml}`
- `nix/*`
- `.cargo/config.toml`
- `rust-toolchain.toml`

Release Notes:

- N/A
a11647d07fJulia Ryan committed on 7/1/2025, 4:14:25 PM· committed by GitHubparent274f2e9
1 file changedLine totals unavailable