main/postmarketos-base-ui: add coreutils as dependency

People running UIs should not have to depend on basic utilities from busybox such as non-portable /usr/bin/env. Total size is 1176KiB, which should be marginal compared to any UI installation.

This came up in a discussion with @sonnyp, related to this issue[[1] in an app he maintains. Like really, we should not be starting something with busybox to save 1MiB 🤷

[1] https://github.com/sonnyp/Tangram/issues/236