ci: failfast when merge conflict exists (frappe#30823)

[skip ci]

.github/helper/install.sh

@@ -2,6 +2,13 @@
 
 set -e
 
+# Check for merge conflicts before proceeding
+python -m compileall -f "${GITHUB_WORKSPACE}"
+if grep -lr --exclude-dir=node_modules "^<<<<<<< " "${GITHUB_WORKSPACE}"
+    then echo "Found merge conflicts"
+    exit 1
+fi
+
 cd ~ || exit
 
 sudo apt-get install redis-server libcups2-dev