[CI] Do not run check-in-tree after pushes to llvm_release_* (#2793)

The check-in-tree tests take about 50 minutes to run compared to <5
minutes for check-out-of-tree.  This causes considerable congestion on
the runners when backports get merged to several llvm_release_*
branches around the same time.

Since the value of these post-push checks is relatively low, disable
them; they will still run on pull requests.

.github/workflows/check-in-tree-build.yml

@@ -9,8 +9,8 @@ name: In-tree build & tests
 on:
   push:
     branches:
+      # This check is expensive; do not run it after pushes to llvm_release_*
       - main
-      - llvm_release_*
     paths-ignore: # no need to check build for:
       - 'docs/**' # documentation
       - '**.md'   # README