Upgrade GitHub CI actions/checkout to v3

This addresses the node v12 deprecation warning in the GitHub CI, caused by the
dependency on actions/checkout@v2.

While actions/checkout@v3 introduces some new features and changes some
defaults, the subset of features that we use should not be affected by this
migration.

The "breaking change" from v2 to v3 can be seen at [0]. Since we are tracking
only v2 without a dot release specified, we are already opting into any breakage
across minor versions, so really the only change of note is the node version
upgrade.

[0]: https://github.com/actions/checkout/compare/v2.4.2...v3.0.0

.github/workflows/main.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Install deps
       run: |
         sudo apt install gettext libncurses5-dev libpcre2-dev python3-pip tmux
@@ -41,7 +41,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Install deps
       run: |
         sudo apt update
@@ -66,7 +66,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Install deps
       run: |
         sudo apt install gettext libncurses5-dev libpcre2-dev python3-pip tmux
@@ -100,7 +100,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Install deps
       run: |
         sudo apt install gettext libncurses5-dev libpcre2-dev python3-pip tmux
@@ -126,7 +126,7 @@ jobs:
     runs-on: macos-latest
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Install deps
       run: |
         sudo pip3 install pexpect