Document cross build step reason & timing in CI

Signed-off-by: Guillaume Tardif <guillaume.tardif@docker.com>

.github/workflows/ci.yml

@@ -53,6 +53,8 @@ jobs:
           path: ~/go/pkg/mod
           key: go-${{ hashFiles('**/go.sum') }}
 
+      # Ensure we don't discover cross platform build issues at release time.
+      # Time used to build linux here is gained back in the build for local E2E step
       - name: Build packages
         run: make -f builder.Makefile cross