Remove redundant work in sanity check of extensions

[Flamefire]

I identified a few things we do in the sanity check of extensions we should not:

• check for things on the whole install tree: folders, rpaths, libraries, CUDA CCs
• Create a module in --sanity-check-only mode

I'm fixing the first one in #5184

The main one here is the 2nd one with this simple diff in extensioneasyblock:

-               with self.fake_module_environment(extra_modules=extra_modules):
+               with self.sanity_check_module_environment(extra_modules=extra_modules):

Not only does this avoid the complex, time-consuming logic of creating a module and manipulating MODULEPATH it might fail if module creation requires information from the build process.
See e.g. easybuilders/easybuild-easyblocks#4122 where get_software_version was used, i.e. the dependency module was required to be loaded.
I added a sanity_check_module_environment context manager to load & unload the sanity check module (real module or fake module)

I'm also starting to remove the extension parameter to those methods, which I did in a PR prior to the 5.3.0 release but took out to get that PR merged without this change.
Reason is to avoid potential mismatches, e.g. by forgetting to pass the parameter.
Ultimately I want to use self.is_extension to allow multiple extension to fail their sanity check step and having a "parallel" source of truth makes that harder.
The cause here is that the failing exts_filter allows to check other extensions but failing commands does not as that aborts directly. Easy to fix when relying on self.is_extension

If there are no objections I'll also remove it from sanity_check_load_module which is similarly simple but requires an update to the easyblocks that use it. Just want to get feedback before doing that extra work.