Only run one build per commit SHA

Every time I push a tag on main, it triggers two builds for the exact
same commit SHA. Instead, we can update our GitHub Actions concurrency
group to use the SHA rather than the ref for disambiguation.

.github/workflows/tests.yml

@@ -1,6 +1,6 @@
 name: re2 Tests
 concurrency:
-  group: "${{github.workflow}}-${{github.ref}}"
+  group: "${{github.workflow}}-${{github.sha}}"
   cancel-in-progress: true
 on:
   workflow_dispatch: