use go-version-file in CI workflows instead of hardcoded versions

Use go-version-file: 'go.mod' so CI automatically picks up the Go
version from go.mod, avoiding future version drift. Reordered
checkout before setup-go in go.yml and e2e.yml so go.mod is
available. Removed the now-unused GO_VERSION env vars.

.github/workflows/sftp-tests.yml

@@ -24,7 +24,6 @@ permissions:
   contents: read
 
 env:
-  GO_VERSION: '1.24'
   TEST_TIMEOUT: '15m'
 
 jobs:
@@ -37,10 +36,10 @@ jobs:
     - name: Checkout code
       uses: actions/checkout@v6
       
-    - name: Set up Go ${{ env.GO_VERSION }}
+    - name: Set up Go
       uses: actions/setup-go@v6
       with:
-        go-version: ${{ env.GO_VERSION }}
+        go-version-file: 'go.mod'
         
     - name: Install dependencies
       run: |