ci: add go-version explicitly since it's not installed on our custom images

Author: james-garner-canonical

.github/workflows/tests.yaml

@@ -21,6 +21,7 @@ jobs:
         uses: actions/setup-go@v6
         with:
           go-version-file: "go.mod"
+          go-version: "stable"
 
       - name: Run tests
         run: |
@@ -54,6 +55,8 @@ jobs:
 
       - name: Setup Go
         uses: actions/setup-go@v6
+        with:
+          go-version: "stable"
 
       - name: Install
         run: |
@@ -89,6 +92,7 @@ jobs:
         uses: actions/setup-go@v6
         with:
           go-version-file: "go.mod"
+          go-version: "stable"
 
       - name: Install
         run: |