.github/workflows/flake-updates: restrict to master

.github/workflows/flake-updates-nixpkgs-update.yml

@@ -5,6 +5,7 @@ on:
 jobs:
   createPullRequest:
     runs-on: ubuntu-latest
+    if: github.ref == 'refs/heads/master' # ensure workflow_dispatch only runs on master
     steps:
       - uses: actions/checkout@v4
       - name: Install Nix

.github/workflows/flake-updates.yml

@@ -7,6 +7,7 @@ on:
 jobs:
   createPullRequest:
     runs-on: ubuntu-latest
+    if: github.ref == 'refs/heads/master' # ensure workflow_dispatch only runs on master
     steps:
       - uses: actions/checkout@v4
       - name: Install Nix