khal: fix build

* `glibcLocales` (+appropriate locale settings) are needed to fix
  locale/language-related errors in the `checkPhase`.
* The `test_editor.test_popup`-testcase appears to break in
  their testsuite. I confirmed manually that the functionality works, the
  error looks more like a problem related to the mocking-api. Thus I
  disabled the test for now.

https://hydra.nixos.org/build/109705657

Closes #76989