ci: check all workspace targets before release builds

[blacks1ne]

Base: f5b671a6

The build job runs cargo check --workspace --locked, and the release-build
matrix is gated on it (needs: build) so three runners aren't spent on a
workspace that doesn't compile.

--workspace checks lib and bin targets only. It does not compile #[cfg(test)]
code, integration tests, benches or examples. A change that breaks a test target
therefore leaves build green, lets the release matrix start, and fails later in
test, where the cause reads as a test failure rather than a compile error.
#613 was exactly that.

--all-targets adds those targets to the same check, sharing the rust-cache
entry the job already restores. Measured on one host, --cpus 10 / CARGO_BUILD_JOBS=6, CARGO_INCREMENTAL=0, off a clean v2.1.0.25:

• a PR-shaped run — touch crates/quil-engine/src/lib.rs, re-check — is
  20/21 s without, 24/24 s with: about +3.5 s
• populating the extra targets after a cache miss costs +34 s once (44 s vs
  10 s) and +264 MB of cache (4.67 GB vs 4.40 GB)
• alternating the two flag sets left the target directory byte-identical, so
  having both forms in play does not thrash the cache

--all-targets has compiled cleanly since #619.

Red checks here are the LFS budget, not this diff.

Validation: git diff --check; the workflow YAML parses.