linter.yml: change default branch from main to master
linter.yml: change default branch from main to master

The main branch in this repo is called master.

Signed-off-by: Roger Luethi <e69c5800a8c6232f48c8f8c6608e2af53bb1dc49@bisdn.de>

bisdn/basebox,bisdn/basebox,bisdn/basebox,bisdn/basebox