Update GitHub Actions

Prepare GitHub Actions checkout for future update where we need to pin the branch as 'master' will not be the default branch anymore.

.github/workflows/lint.yml

@@ -13,6 +13,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
+      with:
+        ref: master
+
     - name: shellcheck
       uses: ludeeus/action-shellcheck@0.4.1
       env:
@@ -22,6 +25,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+      with:
+        ref: master
+
     - name: shfmt
       uses: bltavares/actions/shfmt@master
       env:

.github/workflows/test.yml

@@ -21,7 +21,9 @@ jobs:
           - centos-7-x64
           - centos-8-x64
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@v2
+      with:
+        ref: master
 
     - name: Setup doctl
       uses: digitalocean/action-doctl@v2