cleanup: define required Go version just once (#861)

See https://github.com/ooni/probe/issues/2217

.github/workflows/oohelperd.yml

@@ -14,9 +14,14 @@ jobs:
       contents: write
     steps:
       - uses: actions/checkout@v2
+
+      - name: Get GOVERSION content
+        id: goversion
+        run: echo ::set-output name=version::$(cat GOVERSION)
+
       - uses: actions/setup-go@v1
         with:
-          go-version: "1.18.3"
+          go-version: "${{ steps.goversion.outputs.version }}"
 
       - name: build oohelperd binary
         run: GOOS=linux GOARCH=amd64 CGO_ENABLED=0 go build -o ./CLI/oohelperd-linux-amd64 -v -tags netgo -ldflags="-s -w -extldflags -static" ./internal/cmd/oohelperd