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

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

.github/workflows/alltests.yml

@@ -9,8 +9,14 @@ jobs:
   test:
     runs-on: ubuntu-20.04
     steps:
+      - uses: actions/checkout@v2
+
+      - name: Get GOVERSION content
+        id: goversion
+        run: echo ::set-output name=version::$(cat GOVERSION)
+
       - uses: actions/setup-go@v1
         with:
-          go-version: "1.18.3"
-      - uses: actions/checkout@v2
+          go-version: "${{ steps.goversion.outputs.version }}"
+
       - run: go test -race -tags shaping ./...