update nushell's str collect command to str join

[fdncred]

We've deprecated the nushell command str collect a few months ago. We've replaced it with str join. This PR makes these changes in the activate.nu script.

Thanks for contributing, make sure you address all the checklists (for details on how see development documentation)

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

[fdncred]

These CI failures seem unrelated to this PR. Could someone please take a look?

BTW - One macos test failed yesterday due to timeout. I pushed an empty commit this morning to try and get it to rerun. That's when this other test failure showed up. It's looking like check / test pypy-3.8 - macos-12 is going to fail/timeout again since it's been running more than 1.5 hours now.

[fdncred]

Thanks @gaborbernat!!