This commit makes it possible to use `bump-formula-pr` on linux to
contribute to `homebrew-core` as easily as it is on macOS, but with some
additional steps made before actual bumping of the formula.
Differences in the process (before bumping):
- add `homebrew-core` remote named `homebrew`
- fetch `homebrew/master`
- check if formula exists in `homebrew-core`
- switch to `homebrew/master` ref