Use root as download user for arch linux

The default user is "alpm", which is missing in our /etc/passwd
file. For now just use root instead of creating the user.
We may need to revisit this in the future.

build-pkg-arch

@@ -48,7 +48,7 @@ pkg_cumulate_arch() {
 pkg_install_arch() {
     # Pacman can't handle chroot
     # https://bbs.archlinux.org/viewtopic.php?id=129661
-    (cd $BUILD_ROOT/etc && sed -i "s/^CheckSpace/#CheckSpace/g" pacman.conf)
+    (cd $BUILD_ROOT/etc && sed -i -e "s/^CheckSpace/#CheckSpace/g" -e "s/^DownloadUser/#DownloadUser/" pacman.conf)
     # -d -d disables deps checking
     ( cd $BUILD_ROOT && chroot $BUILD_ROOT pacman -U --overwrite '*' -d -d --noconfirm .init_b_cache/$PKG.$PSUF 2>&1 || touch $BUILD_ROOT/exit ) | \
 	perl -ne '$|=1;/^(warning: could not get filesystem information for |loading packages|looking for inter-conflicts|looking for conflicting packages|Targets |Total Installed Size: |Net Upgrade Size: |Proceed with installation|checking package integrity|loading package files|checking for file conflicts|checking keyring|Packages \(\d+\)|:: Proceed with installation|:: Processing package changes|checking available disk space|installing |upgrading |warning:.*is up to date -- reinstalling|Optional dependencies for|    )/||/^$/||print'