ci: pin lint job to stable Go

Without go-version, setup-go defaulted to 1.24 in the lint job, which
is below the go.mod 1.25.0 floor; with GOTOOLCHAIN=local preventing
auto-upgrade, golangci-lint failed to load packages.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dunglas

.github/workflows/ci.yaml

@@ -19,6 +19,8 @@ jobs:
       uses: actions/checkout@v6
     - name: Set up Go
       uses: actions/setup-go@v6
+      with:
+        go-version: stable
     - name: Lint Go Code
       uses: golangci/golangci-lint-action@v9