pkgix: Permit multiple `source_prefix_rc` calls

Previously `source_prefix_rc` was only permitted to run once. This
commit moves `source_prefix_rc` after the check that a prefix
environment had already been set, to take advantage of this check
without having to rely on a check specific to `source_prefix_rc`.

Signed-off-by: Marco Elver <me@marcoelver.com>