We don't want to silently ignore failing commands.
Also, while we're editing the `tap-new` generated `azure-pipelines.yml`
also add the changes missed here from #5600.
- ensure that `HOMEBREW_CELLAR` is always created on `install`.
- remove the need for a special `PRUNEABLE_DIRECTORIES` variable
- reuse values from existing variables and get `uniq`s.
Use the environment variables set by `brew test-bot`. Eventually we'll
disable Travis CI running CodeCov so move `TRAVIS` references to
`HOMEBREW_TRAVIS_CI` so it doesn't need whitelisted.
Also, fix `azure-pipelines.yml` so it's testing the correct version of
Homebrew/brew (the one checked out in the `pwd`).