micro-activate: generate machine-id randomly

D-Bus is supposed to (?) use it to decide whether it can use FD passing,
shared memory, etc. and while we do a lot of cross-domain magic it's not
quite seamless :) so let's not reuse the host one.

munix

@@ -195,7 +195,6 @@ bwrap --unshare-all --share-net \
   --symlink "$MICROVM_CLOSURE/sw/bin/env" /usr/bin/env \
   --symlink "$MICROVM_CLOSURE" /run/current-system \
   --ro-bind /nix/store /nix/store \
-  --file 12 /run/machine-id \
   --file 13 /run/resolv.conf \
   --dir "$XDG_RUNTIME_DIR" \
   --setenv PATH "/run/munix/muvm:/run/munix/passt:$MICROVM_CLOSURE/sw/bin" \
@@ -207,5 +206,4 @@ bwrap --unshare-all --share-net \
     -e MICROVM_CLOSURE="$MICROVM_CLOSURE" \
     -e MICROVM_UID="$MICROVM_UID" -e MICROVM_GID="$MICROVM_GID" \
     -i -t "${MICROVM_COMMAND[@]}" \
-  12< /etc/machine-id \
   13< /etc/resolv.conf