Enable --offline installs from pkgs_dirs

[jaimergp]

Description

Closes #396

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[jaimergp]

Needs conda/conda#13507

[jaimergp]

I can't reproduce locally:

FAILED tests/cli/test_main_clean.py::test_clean_tarballs - AssertionError: assert False
 +  where False = has_pkg('zlib', [])
 +    where [] = _get_tars('/var/folders/mm/pltwc2yj1jx192t6dzy9zsrr0000gn/T/1dd3 dcafbe/pkgs')
FAILED tests/cli/test_main_clean.py::test_clean_all[True] - AssertionError: assert False
 +  where False = has_pkg('zlib', [PosixPath('/var/folders/mm/pltwc2yj1jx192t6dzy9zsrr0000gn/T/9650 cbfdae/pkgs/cache')])
FAILED tests/cli/test_main_clean.py::test_clean_all_mock_lstat[True] - AssertionError: assert False
 +  where False = has_pkg('zlib', [PosixPath('/var/folders/mm/pltwc2yj1jx192t6dzy9zsrr0000gn/T/beba ecfdab/pkgs/cache')])

It's like that pkgs cache is not functional for those tests. This only happens on macOS, so I wonder if there's a side effect introduced by other tests.

[jaimergp]

Actually these are also failing on main, so I'd say it's not related to this PR at all.