ci: Print verbose build error message in test-each-commit

.github/ci-test-each-commit-exec.py

@@ -43,7 +43,11 @@ def main():
         # Tolerate unused member functions in intermediate commits in a pull request
         "-DCMAKE_CXX_FLAGS=-Wno-error=unused-member-function",
     ])
-    run(["cmake", "--build", build_dir, "-j", str(num_procs)])
+
+    if run(["cmake", "--build", build_dir, "-j", str(num_procs)], check=False).returncode != 0:
+        print("Build failure. Verbose build follows.")
+        run(["cmake", "--build", build_dir, "-j1", "--verbose"])
+
     run([
         "ctest",
         "--output-on-failure",