fix: handle symlink to charts directory other than a local subdirectory

[smlx]

Checklist

• Affected Issues have been mentioned in the Closing issues section
• Documentation has been written/updated
• PR title is ready for changelog and subsystem label(s) applied

Part of my development workflow is to point the charts directory symlink used by make kind/retest at a local charts directory that is outside of my lagoon working directory. Currently that doesn't work.

This change allows kind/retest to follow that symlink even if it is not pointing at a subdirectory of the lagoon working directory.

If the charts symlink is pointing somewhere other than a subdirectory of
the local lagoon working tree then readlink gets confused. The problem
is described here, which is also where I got the solution:
https://superuser.com/a/1217241

Closing issues

n/a