Also fix issue where workflow fails on pull requests when the
user who made the pull request doesn't have the secrets set.
Also allow other workflows to be triggered manually.
Also don't trigger any workflows on pull requests to master, as
there shouldn't be any pull requests against master.
InfiniSim PR https://github.com/InfiniTimeOrg/InfiniSim/pull/10 adds
support for screenshots in png/bmp format using `libpng` submodule.
This new submodule must be added to the lv_sim workflow as well.
