archlinux: fix package upgrade command handling

[baodrate]

Proposed Commit Message

archlinux: fix package upgrade command handling

pacman uses `-u` instead of `upgrade` to trigger a system upgrade, fix
the command handling so this is properly accounted for. as is, the
resulting command attempts to install a (non-existent) `upgrade` package

Additional Context

Pretty straightforward. Arch's pacman uses -u instead of upgrade (see man 8 pacman). Without this change I get an error when package_upgrade: true:

Cloud-init v. 20.4 running 'single' at Tue, 12 Jan 2021 00:37:17 +0000. Up 3995.82 seconds.
:: Synchronizing package databases...
error: target not found: upgrade
 core is up to date
 extra is up to date
 community is up to date
2021-01-12 00:37:17,927 - util.py[WARNING]: Package upgrade failed
2021-01-12 00:37:17,928 - cc_package_update_upgrade_install.py[WARNING]: 1 failed with exceptions, re-raising the last one
2021-01-12 00:37:17,928 - util.py[WARNING]: Running module package-update-upgrade-install (<module 'cloudinit.config.cc_package_update_upgrade_install' from '/usr/lib/python3.9/site-packages/cloudinit/config/cc_package_update_upgrade_install.py'>) failed
2021-01-12 00:37:17,929 - main.py[WARNING]: Ran package-update-upgrade-install but it failed!

Test Steps

I think some tests for this would be good to avoid regressions but I'm pretty unfamiliar with cloud-init and moreso with the project's testing setup. Any help would be appreciated

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly