validate:launcher: Add a GST_VALIDATE_LAUNCHER_MAIN_DIR env variable

This is generally usefull so we do not have to pass -M every time we launch the launcher

And it adds support for nesting launcher calls always respecting the provided main directory

+ Fix some new pep8 errors
3 jobs for launcher_fixes in 8 minutes and 8 seconds (queued for 4 minutes and 57 seconds)
Status Name Job ID Coverage
  Preparation
passed manifest #54100

00:00:28

 
  Build
passed build fedora x86_64 #54101

00:03:49

 
  Full Builds
passed build android arm64 api28 #54102

00:03:50