images/archlinux: run chroot in new PID namespace

gpg-agent remains after exit, and causes umount to fail.

Mirrors what was done in arch-chroot:
2be79c6259

images/archlinux/genimg

@@ -34,12 +34,14 @@ pacstrap -G root base base-devel git mercurial openssh mkinitcpio linux syslinux
 
 run_root() {
     local cmd="$@"
-    chroot $root /bin/bash -c "$cmd"
+    # Run in a PID namespace to tear down processes remaining in the chroot on
+    # exit (e.g. gpg-agent)
+    unshare --fork --pid chroot $root /bin/bash -c "$cmd"
 }
 
 run_normal() {
     local cmd="$@"
-    chroot $root sudo -u build -g build \
+    unshare --fork --pid chroot $root sudo -u build -g build \
         /bin/bash -c "cd /home/build && $cmd"
 }