feat: Use erikmd/docker-coq-action@v1

Author: erikmd

.github/workflows/build-coq-demo.yml

@@ -34,7 +34,7 @@ jobs:
     steps:
     # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
     - uses: actions/checkout@v2
-    - uses: erikmd/docker-coq-action@master
+    - uses: erikmd/docker-coq-action@v1
       with:
         opam_file: 'coq-demo.opam'
         coq_version: ${{ matrix.coq_version }}
@@ -52,7 +52,7 @@ jobs:
       max-parallel: 4
     steps:
     - uses: actions/checkout@v2
-    - uses: erikmd/docker-coq-action@master
+    - uses: erikmd/docker-coq-action@v1
       with:
         opam_file: './coq-demo.opam'
         custom_image: ${{ matrix.image }}