push-build: Fix directory listing for fast builds
push-build: Fix directory listing for fast builds

Ensure we check for "fast" (linux/amd64-only) build artifacts in the
/fast subdirectory instead of the "root" of the build directory when
attempting to write build version markers.

Signed-off-by: Stephen Augustus <f96984fef9637085f0559f1484dfd4e99545cd6e@vmware.com>

kubernetes/release,kubernetes/release