makechrootpkg: make the -U option work for the first time ever

The -U option was initially introduced in commit
cda9cf436b in order to enable running
makechrootpkg as root, delegating to another, manually selected, user to
perform various non-root tasks (given that makepkg was modified to throw
fatal errors when run as root without the option of --asroot to disable
that). However, it was only ever implemented for the --verifysource
option outside of the chroot, and the builduser inside the chroot is
created with the same uid as the makechrootpkg invoker. It needs to run
as the same uid, because it needs rw access to $startdir and $SRCDEST!
Additionally this lets the invoking user more easily inspect the build
directory in case of problems...

The correct solution for this is to properly implement the initial
intention of the -U option, and make it override the autodetection of
the "invoking user" which is normally done by inspecting $SUDO_USER.
This is then used as the single source of truth for "who am I pretending
to be".

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Levente Polyak <anthraxx@archlinux.org>

makechrootpkg.in

@@ -172,8 +172,8 @@ prepare_chroot() {
 	(( keepbuilddir )) || rm -rf "$copydir/build"
 
 	local builduser_uid builduser_gid
-	builduser_uid="${SUDO_UID:-$UID}"
-	builduser_gid="$(id -g "$builduser_uid")"
+	builduser_uid="$(id -u "$makepkg_user")"
+	builduser_gid="$(id -g "$makepkg_user")"
 	local install="install -o $builduser_uid -g $builduser_gid"
 	local x