ci: make fmt-check during ci

[mnahkies]

Edit: repurposed to check fmt during CI

• add fmt job to the android github actions workflow
• re-introduce hint error message on make fmt-check failure
• update readme

stacked on #696 this change simply runs the new make lint command to reformat the code.

I wasn't sure how annoying to review this would be, and so figured there is a decent chance a maintainer might prefer to run themselves (essentially replacing this PR).

[bradfitz]

It should probably also have CI coverage to prevent regressions in the future.

Edit: I should've read #696 first :)

[mnahkies]

It should probably also have CI coverage to prevent regressions in the future.

Edit: I should've read #696 first :)

Given the diff isn't as bad to review as I'd guessed it might be, I'd be happy to fold this into #696 now that it's been demonstrated to do its job in CI correctly.

[mnahkies]

@bradfitz I've rebased this on main and repurposed to only add the make fmt-check CI job now that #696 is merged 🙏