semaphore: keep build settings in the repository

It should make it easier to tweak them without having access to
SemaphoreCI itself (I'm glad I'm an admin there now but it shouldn't
be necessary to be an admin to add a couple of kludges :-)). More
importantly, changes to the settings will go through review and
be tested before they're applied globally potentially breaking
Semaphore as it happened two days ago.

I'll point Semaphore CI to these scripts once the PR is merged.