Add condition of having > 0 commits in past 24hrs

If there have been no changes in the past 24 hours, then it's
pointless to run this full build and test. We can save resources by
skipping it.

Author: mhucka

.github/workflows/ci-nightly-build-test.yaml

@@ -7,6 +7,9 @@
 # failures that might be missed when skipping the long-running tests or using
 # caching to speed up the CI runs.
 #
+# For efficiency, it checks if there have been any commits in the past 24 hrs
+# and does not proceed if there have been none.
+#
 # This workflow also can be invoked manually via the "Run workflow" button at
 # https://github.com/tensorflow/quantum/actions/workflows/ci-build-checks.yaml
 # ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -52,8 +55,27 @@ concurrency:
   group: ${{github.workflow}}-${{github.event.pull_request.number||github.ref}}
 
 jobs:
+  Decision:
+    runs-on: ubuntu-24.04
+    outputs:
+      run: ${{steps.commits.outputs.count > 0}}
+    steps:
+      - name: Check out a sparse copy of the git repo for TFQ
+        uses: actions/checkout@v4
+        with:
+          sparse-checkout: .
+
+      - name: Get number of commits in the last 24 hrs
+        id: commits
+        run: |
+          set -x
+          count=$(git log --oneline --since '24 hours ago' | wc -l)
+          echo "count=$count" >> "$GITHUB_OUTPUT"
+
   Nightly:
+    if: needs.Decision.outputs.run == 'true'
     name: Build and test
+    needs: Decision
     runs-on: ubuntu-20.04
     steps:
       - name: Check out a copy of the TFQ git repository