archlinux: create mnt_archlinux_dvd directory before mount

It was created in 00_prepare.sh, but only in case of downloading iso
image, which can be already done by qubes-builder.

scripts_archlinux/01_install_core.sh

@@ -3,6 +3,7 @@
 set -e
 
 echo "Mounting archlinux install system into mnt_archlinux_dvd..."
+mkdir -p mnt_archlinux_dvd
 sudo mount $CACHEDIR/root-image.fs mnt_archlinux_dvd
 
 echo "Creating chroot bootstrap environment"

scripts_archlinux/04_install_qubes.sh

@@ -3,6 +3,7 @@
 set -e
 
 echo "Mounting archlinux install system into mnt_archlinux_dvd..."
+mkdir -p mnt_archlinux_dvd
 sudo mount $CACHEDIR/root-image.fs mnt_archlinux_dvd
 
 # Note: Enable x86 repos