Don't use priority for module use

[Flamefire]

See https://github.com/easybuilders/easybuild-framework/pull/3557, especially:

We have a function prepend_module_path with an optional priority. Hence if we don't pass a priority then this function will not "prepend" the path

Especially for LMod we should not use the priority at all. Instead we can simply implement use, unuse and prepend by modifying MODULEPATH as is done for use in #3557

Note: As a side-effect from #3557 prepend_module_path without a priority is already working as it should now.

[boegel]

@Flamefire Is this fixed with the changes in #3636? If so, can you add a fixes #3631 in the PR description there?

[Flamefire]

Yes and done