gha: buildkit: make sure expected Go version is installed

The buildkit workflow uses Go to determine the version of Buildkit to run
integration-tests for. It currently uses on the default version that's
installed on the GitHub actions runners (1.21.13 currently), but this fails
if the go.mod/vendor.mod specify a higher version of Go as required version.

If this fails, the BUILDKIT_REF and REPO env-vars are not set / empty,
resulting in the workflow checking out the current (moby) repository instead
of buildkit, which fails.

This patch adds a step to explicitly install the expected version of Go.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

.github/workflows/buildkit.yml

@@ -102,6 +102,11 @@ jobs:
         uses: actions/checkout@v4
         with:
           path: moby
+      -
+        name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: ${{ env.GO_VERSION }}
       -
         name: BuildKit ref
         run: |