set go's mod cache env var to a directory we can write to

Author: zerbitx

.github/workflows/e2e-tests.yaml

@@ -54,6 +54,12 @@ jobs:
       uses: actions/setup-go@v5
       with:
         go-version: "1.22"
+        
+    - name: Set up Go module cache directory
+      run: |
+        export GOMODCACHE="${{ github.workspace }}/.gomodcache"
+        mkdir -p "$GOMODCACHE"
+        echo "GOMODCACHE=$GOMODCACHE" >> $GITHUB_ENV
 
     - name: Build devspacehelper
       run: |