workflows/tests: skip update-test unless on a PR.
workflows/tests: skip update-test unless on a PR.

If this is a `master` build and it was merged: this is a flaky test we
can't do anything about.

nandub/brew,vitorgalvao/brew,nandub/brew,konqui/brew,sjackman/homebrew,sjackman/homebrew,nandub/brew,MikeMcQuaid/brew,reitermarkus/brew,vitorgalvao/brew,JCount/brew,mahori/brew,claui/brew,reitermarkus/brew,MikeMcQuaid/brew,vitorgalvao/brew,konqui/brew,MikeMcQuaid/brew,nandub/brew,konqui/brew,MikeMcQuaid/brew,reitermarkus/brew,Homebrew/brew,sjackman/homebrew,EricFromCanada/brew,claui/brew,Homebrew/brew,EricFromCanada/brew,konqui/brew,mahori/brew,claui/brew,EricFromCanada/brew,JCount/brew,vitorgalvao/brew,reitermarkus/brew,EricFromCanada/brew,sjackman/homebrew,JCount/brew,claui/brew,Homebrew/brew,Homebrew/brew,JCount/brew,mahori/brew,mahori/brew