github: Update to use 'checkout@v4' action to avoid Node.js warnings

The following warning is displayed multiple times in the "Annotations"
section of a Github action that runs the main workflow:

~~~
Node.js 12 actions are deprecated. Please update the following actions
to use Node.js 16: actions/checkout@v2. For more information see:
https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/.
~~~

Updating the main workflow to use checkout@v4 elimiates this warning.

Signed-off-by: Jason Gerecke <[email protected]>

.github/workflows/main.yml

@@ -55,7 +55,7 @@ jobs:
           gcc -print-file-name=plugin
 
       - name: Checkout the repo
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       - name: Download and extract v${{ matrix.kernel }} kernel
         run: |