home-manager: prioritize -I parameters

In 176e455 the order between the action of `-I` parameters getting
added to `EXTRA_NIX_PATH` and the action of a static path getting
added to `EXTRA_NIX_PATH` was reversed, also reversing the order of
`-I` parameters and the static `-I home-manager=...` leading to the
static `-I home-manager=...` to always come before any of the `-I`
parameters to later calls to Nix commands.

This made it impossible to override the static Home Manager path when
calling the home-manager tool with `-I home-manager=...`. This was
previously possible.

(cherry picked from commit edf9cf6523)

home-manager/home-manager

@@ -883,7 +883,6 @@ COMMAND=""
 COMMAND_ARGS=()
 FLAKE_ARG=""
 
-setHomeManagerNixPath
 setHomeManagerPathVariables
 
 while [[ $# -gt 0 ]]; do
@@ -979,6 +978,8 @@ while [[ $# -gt 0 ]]; do
     esac
 done
 
+setHomeManagerNixPath
+
 if [[ -z $COMMAND ]]; then
     doHelp >&2
     exit 1