Move metadata validation to CI build step

katversion needs git, so the validation also needs git.  Move the
metadata validation to the build step, rather than installing git
again in the publish step.  The dist files from the build step are
the ones that get published, so OK to move the check there.

.gitlab-ci.yml

@@ -33,6 +33,7 @@ build wheel:
     # TODO:  consider moving git installation to ska-python-buildenv Dockerfile
     - apt-get -y update
     - apt-get install -y git --no-install-recommends
+    - scripts/validate-metadata.sh
     - python setup.py sdist bdist_wheel
   artifacts:
     paths:
@@ -88,8 +89,6 @@ publish to nexus:
     TWINE_USERNAME: $TWINE_USERNAME
     TWINE_PASSWORD: $TWINE_PASSWORD
   script:
-    # Check metadata requirements
-    - scripts/validate-metadata.sh
     - pip install twine
     - twine upload --repository-url $PYPI_REPOSITORY_URL dist/*
   only: