meson: correctly differentiate $profiledir and $sysconfdir/profile.d

The configured sysconfdir is used to look for nix.conf, so it needs
to be /etc, and not $out/etc, so we separate out the place where shell
profile files are installed, which is the only other place sysconfdir is
at all used.

See https://git.lix.systems/lix-project/lix/issues/231#issuecomment-1989
for more info.

Change-Id: Idbed8ba82e711b8a9d6b6127904befa27d58e279

scripts/meson.build

@@ -20,7 +20,7 @@ foreach rc : [ '.sh', '.fish', '-daemon.sh', '-daemon.fish' ]
     input : 'nix-profile' + rc  + '.in',
     output : 'nix' + rc,
     install : true,
-    install_dir : sysconfdir / 'profile.d',
+    install_dir : profile_dir,
     install_mode : 'rw-r--r--',
     configuration : {
       'localstatedir': state_dir,