Add local build Maven profile

[JanWittler]

In the GitHub actions, currently, to build against the local build of the core and domains repositories sed is used to replace the update site location with a reference to a local build.
This is hard to understand and error-prone.
Furthermore, when testing local changes against the domains or applications repository, the same workaround has to be used to reference the local copy instead of the update site.

Both issues can be solved by adding a local build Maven profile. The suggested change is to add a profile with a configurable path to the local version instead of to the update site. This change should not modify the default behavior but only occur when explicitly enabled.