refactor(ci): define single go version

.github/workflows/ci.yaml

@@ -6,6 +6,9 @@ on:
     branches:
       - main
 
+env:
+  GO_VERSION: '1.21'
+
 jobs:
   tests:
     name: "Tests"
@@ -14,7 +17,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-go@v4
         with:
-          go-version: '1.21'
+          go-version: ${{ env.GO_VERSION }}
       - run: . scripts/test
   integration-tests:
     name: "Integration tests"
@@ -25,5 +28,5 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-go@v4
         with:
-          go-version: '1.21'
+          go-version: ${{ env.GO_VERSION }}
       - run: . integration/install_specific_version