Improve test-release CI job runtime

As we add more jobs and have existing ones perform more work, CI slows
down. One of the main offenders is the 'test-release' job, which is
found to have a both cold start runtime but also doesn't play
particularly nice with our caching infrastructure. This has potential
trickle down effects on other jobs, which may only get scheduled on
runners once others have finished (because there exist overall limits on
the number of runners in use), potentially delaying overall CI time
doubly.
The slowness of this job seems to come down to two things:
1) our release builds use LTO etc., making linking a very lengthy
   process
2) it turns out that release builds are not incremental by default [0],
   causing a lot of recompilation to happen, despite our efforts of
   caching intermediate artifacts

To fix these issues, we disable the link time optimization step in CI
and enable incremental compilation. Doing so reduces runtime from >=3min
down to ~1min.

[0] https://doc.rust-lang.org/cargo/reference/profiles.html#release

Signed-off-by: Daniel Müller <[email protected]>

.github/workflows/test.yml

@@ -320,7 +320,11 @@ jobs:
     - uses: actions/checkout@v4
     - uses: dtolnay/rust-toolchain@stable
     - uses: Swatinem/rust-cache@v2
-    - run: cargo test --workspace --release
+    - env:
+        # Support incremental compilation here to not penalize CI compile times too much.
+        CARGO_PROFILE_RELEASE_INCREMENTAL: true
+        CARGO_PROFILE_RELEASE_LTO: false
+      run: cargo test --workspace --release
   test-miri:
     name: Test with Miri
     runs-on: ubuntu-latest