CI: Upgrade actions (#82)

This PR upgrades CI actions to their latest versions.

At the very least, the codecov action is necessary because otherwise PRs
stall on codecov upload step (example:
#81)

.github/workflows/go.yml

@@ -23,15 +23,15 @@ jobs:
 
     steps:
     - name: Setup Go
-      uses: actions/setup-go@v2
+      uses: actions/setup-go@v5
       with:
         go-version: ${{ matrix.go }}
 
     - name: Checkout code
-      uses: actions/checkout@v2
+      uses: actions/checkout@v4
 
     - name: Load cached dependencies
-      uses: actions/cache@v1
+      uses: actions/cache@v4
       with:
         path: ~/go/pkg/mod
         key: ${{ runner.os }}-go-${{ hashFiles('**/go.sum') }}
@@ -49,4 +49,6 @@ jobs:
       run: make cover
 
     - name: Upload coverage to codecov.io
-      uses: codecov/codecov-action@v1
+      uses: codecov/codecov-action@v4
+      env:
+        CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}