Add env var to toggle post-merge hook

Author: cheap-glitch

src/post-merge

@@ -7,6 +7,8 @@ SOURCE_DIR="$(dirname "$(realpath "${BASH_SOURCE[0]}")")"
 # shellcheck source=src/helpers/is-changed.sh
 source "${SOURCE_DIR}/helpers/is-changed.sh"
 
+if [[ ${GIT_GRAPNEL_AUTO_INSTALL:-0} -eq 0 ]]; then exit 0; fi
+
 if [[ -f package-lock.json ]] && is_changed package-lock.json; then
 	npm clean-install || npm install
 fi