build,circleci: change exclude pattern

Even when configured to only build PR, CircleCI always build the default branch
`only: /pull.*/` seems like a good idea, but it fails when:
- you are testing on the same repo, because github doesn't create a pull branch
- when your repo is configured, because it see the push on your repo,
prevent building this commit because it doesn't match the regexp, then when you
do the PR CircleCI doesn't work
In the end what we really want is `ignore: master`

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>

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