GitHub CI: fix Go version

Fix Go version in YAML: use quotes to mark <1.20> as string instead of
being interpreted as float as <1.2>.

.github/workflows/go.yml

@@ -19,7 +19,7 @@ jobs:
     - name: Set up Go 1.20
       uses: actions/setup-go@v4
       with:
-        go-version: 1.20
+        go-version: '1.20'
       id: go
 
     - name: Get dependencies