Upgrade GitHub Actions

https://github.com/actions/checkout/releases

Signed-off-by: Christian Clauss <cclauss@me.com>

Author: cclauss

.github/workflows/build.yml

@@ -18,7 +18,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
         with:
           submodules: recursive
 

.github/workflows/main.yml

@@ -19,7 +19,7 @@ jobs:
 
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - name: install nightly rust
         run: |