CI: use v2 of actions/checkout

similarly to the documentation:
    <https://github.com/actions/checkout#usage>

Author: johnyf

.github/workflows/main.yml

@@ -80,7 +80,7 @@ jobs:
           TAG_REF_NAME: ${{ github.ref }}
           REPOSITORY_NAME: ${{ github.repository }}
       ## Get TLAPS repository checked out.
-      - uses: actions/checkout@master
+      - uses: actions/checkout@v2
       ## Setup OCaml environment.
       - uses: ocaml/setup-ocaml@v2
         with:

.github/workflows/pr.yml

@@ -20,7 +20,7 @@ jobs:
           DOWNLOADS: tlaps-1.5.0-x86_64-linux-gnu
     steps:
       ## Get TLAPS repository checked out.
-      - uses: actions/checkout@master
+      - uses: actions/checkout@v2
       ## Setup OCaml environment.
       - uses: ocaml/setup-ocaml@v2
         with: