meson: make user $PATH configurable

This partially reverts db11487d (the logic to
calculate the correct value is removed, we always use the same setting as for
the system manager). Distributions have an easy mechanism to override this if
they wish.

I think making this configurable is better, because different distros clearly
want different defaults here, and making this configurable is nice and clean.
If we don't make it configurable, distros which either have to carry patches,
or what would be worse, rely on some other configuration mechanism, like
/etc/profile. Those other solutions do not apply everywhere (they usually
require the shell to be used at some point), so it is better if we provide
a nice way to override the default.

Fixes  #13469.