github: fix lack of v prefix on build_args

Change-Id: I6d91e12e05d17a07f08548e8c71c7cdc9dfef82d
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/5600
Reviewed-by: Marcel van Lohuizen <mpvl@google.com>
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 5944d49..c258d2f 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -49,5 +49,5 @@
           tag_with_sha: false
           target: cue
           always_pull: true
-          build_args: GOLANG_VERSION=${{ env.GOLANG_VERSION }},CUE_VERSION=${{ env.CUE_VERSION }}
+          build_args: GOLANG_VERSION=${{ env.GOLANG_VERSION }},CUE_VERSION=v${{ env.CUE_VERSION }}
           add_git_labels: true