add support for environment variables in set_environment of module generators

[lexming]

This PR enhances the set_environment methods in module generators to handle contents of variables that have references to shell environment variables (i.e. $ENV_VAR).

The effects of this PR are better seen in test_env in test/framework/module_generator.py, which is heavily enhanced to test a lot more cases. See test/framework/module_generator.py#L956

In practice, this now allows the following in modextravars:

modextravars = {
    'SOME_VARIABLE_PYTHON': '$EBROOTPYTHON/bin/python',
    'SOME_VARIABLE_CONFIG': '$HOME/.config/file',
}

Additionally, in the case of ModuleGeneratorLua, the static template PATH_JOIN_TEMPLATE used to construct pathJoin commands in Lua is replaced with a more powerful _path_join_cmd method that properly splits the components of the path:

# OLD
pathJoin(root, "foo/bar")
# NEW
pathJoin(root, "foo", "bar")

Companion PR in easyconfigs:

• replace setenv commands with environment variables in module footers with modextravars in easyconfigs for HIP, JupyterLab, mujoco-py, Triton easybuild-easyconfigs#22801

[smoors]

after a quick discussion with @lexming he will add an option to opt out of resolving the variables

[lexming]

@smoors as discussed, resolution of shell variables is enabled by default and can be disabled with an extra option in modextravars called shell_vars:

modextravars = {
    'VARIABLE_NAME': '$HOME/path',
    'PASSWORD_VAR': {'value': 'dijd$12h12hr', 'shell_vars': False},
}

Updated unit tests accordingly.

[smoors]

lgtm

[boegel]

lgtm