ci(buildkit): match moby go version for buildkit tests

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

.github/workflows/buildkit.yml

@@ -13,6 +13,7 @@ on:
   pull_request:
 
 env:
+  GO_VERSION: "1.20.5"
   DESTDIR: ./build
 
 jobs: