workflow: Add top-level permissions

Set:
```
permissions:
  contents: read
```
as the default top-level permissions explicitly
to conform to recommended security practices e.g.
https://github.com/ossf/scorecard/blob/main/docs/checks.md#token-permissions

src/runtime/pkg/govmm/.github/workflows/main.yml

@@ -1,5 +1,9 @@
 on: ["pull_request"]
 name: Unit tests
+
+permissions:
+  contents: read
+
 jobs:
   test:
     strategy: