CI: Add cargo tests to aarch64-apple-darwin

This adds running of cargo's tests to the aarch64-apple-darwin job. The
reason for this is that tier-1 targets are ostensibly supposed to run
tests for host tools, but we are not doing that here. We do have fairly
good coverage in Cargo's CI, but we don't have much coverage of beta or
stable. I think it would be good to have a fallback here.

src/ci/github-actions/jobs.yml

@@ -459,7 +459,9 @@ auto:
 
   - name: aarch64-apple
     env:
-      SCRIPT: ./x.py --stage 2 test --host=aarch64-apple-darwin --target=aarch64-apple-darwin
+      SCRIPT: >
+        ./x.py --stage 2 test --host=aarch64-apple-darwin --target=aarch64-apple-darwin &&
+        ./x.py --stage 2 test --host=aarch64-apple-darwin --target=aarch64-apple-darwin src/tools/cargo
       RUST_CONFIGURE_ARGS: >-
         --enable-sanitizers
         --enable-profiler