ci: fix snapshot publishing

[alexeagle]

Seems like a bug: by ssh'ing into a job, I see that CIRCLE_PULL_REQUEST is set to a bogus value on non-pull-request builds.

[ngbot]

I see that you just added the PR action: merge label, but the following checks are still failing:
     missing required label: "cla: yes"
     status "ci/circleci: build-packages-dist" is pending
     status "ci/circleci: test" is pending
     status "continuous-integration/travis-ci/push" is pending
     status "ci/circleci: lint" is pending
     status "code-review/pullapprove" is pending
     status "continuous-integration/travis-ci/pr" is pending
     1 pending code review

If you want your PR to be merged, it has to pass all the CI checks.

If you can't get the PR to a green state due to flakes or broken master, please try rebasing to master and/or restarting the CI job. If that fails and you believe that the issue is not due to your change, please contact the caretaker and ask for help.

[angular-automatic-lock-bot]

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