Fix macOS jobs on Cirrus CI
Fix macOS jobs on Cirrus CI

These jobs fail now, all with the same error:

```
$ brew install "gettext" && brew link --force "gettext"
Warning: gettext 0.20.1 is already installed and up-to-date
To reinstall 0.20.1, run `brew reinstall gettext`
Warning: Refusing to link macOS provided/shadowed software: gettext
If you need to have gettext first in your PATH run:
  echo 'export PATH="/usr/local/opt/gettext/bin:$PATH"' >> ~/.bash_profile
For compilers to find gettext you may need to set:
  export LDFLAGS="-L/usr/local/opt/gettext/lib"
  export CPPFLAGS="-I/usr/local/opt/gettext/include"
```

(https://cirrus-ci.com/task/4695796725579776?command=install#L1865)

Apparently macOS images now ship with some version of gettext, so
Homebrew fails to install its own. We follow the recommendation and set
the environment variables to get around this.

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