ci(workflow): don't run jobs twice on PRs

.github/workflows/push.yml

@@ -1,5 +1,12 @@
-on: [push, pull_request]
-name: ShellCheck
+on:
+  push:
+    branches:
+    - master
+  pull_request:
+    branches:
+    - master
+
+name: Lint
 jobs:
   shellcheck:
     runs-on: ubuntu-latest
@@ -9,3 +16,11 @@ jobs:
       uses: ludeeus/action-shellcheck@0.0.1
       with:
         args: openvpn-install.sh -e SC1091,SC2164,SC2034,SC1072,SC1073,SC1009
+  shfmt:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - name: shfmt
+      uses: bltavares/actions/shfmt@master
+      env:
+        SHFMT_ARGS: -d