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

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

.github/workflows/gosec.yml

@@ -11,14 +11,18 @@ jobs:
     runs-on: ubuntu-20.04
     env:
         GO111MODULE: on
-    strategy:
-      matrix:
-        go:
-          - "1.18.3"
     steps:
     - name: Checkout Source
       uses: actions/checkout@v2
 
+    - name: Get GOVERSION content
+      id: goversion
+      run: echo ::set-output name=version::$(cat GOVERSION)
+
+    - uses: actions/setup-go@v1
+      with:
+        go-version: "${{ steps.goversion.outputs.version }}"
+
     - name: Run Gosec security scanner
       continue-on-error: true # TODO(https://github.com/ooni/probe/issues/2180)
       uses: securego/gosec@master