ci(convert): add missing substitute var (#445)

"""
cache miss because layer definition was changed
error: layer needs to be rebuilt before publishing: alpine
error: exit status 1
"""

Signed-off-by: Ramkumar Chinchani <rchincha@cisco.com>

.github/workflows/convert.yaml

@@ -36,7 +36,7 @@ jobs:
           TEMPDIR=$(mktemp -d)
           stacker convert --docker-file Dockerfile --output-file stacker.yaml --substitute-file stacker-subs.yaml
           stacker build -f stacker.yaml --substitute-file stacker-subs.yaml --substitute IMAGE=alpine --substitute STACKER_VOL1="$TEMPDIR"
-          stacker publish -f stacker.yaml --substitute-file stacker-subs.yaml --substitute IMAGE=alpine --skip-tls --url docker://${REGISTRY_URL} --layer alpine --tag latest
+          stacker publish -f stacker.yaml --substitute-file stacker-subs.yaml --substitute IMAGE=alpine --substitute STACKER_VOL1="$TEMPDIR" --skip-tls --url docker://${REGISTRY_URL} --layer alpine --tag latest
           rm -f stacker.yaml stacker-subs.yaml
           stacker clean
           cd $GITHUB_WORKSPACE