runinstallvm: Fix image path

Signed-off-by: Marc Zyngier <maz@kernel.org>
diff --git a/runinstallvm.sh b/runinstallvm.sh
index 73d5cf0..ea037db 100755
--- a/runinstallvm.sh
+++ b/runinstallvm.sh
@@ -5,7 +5,7 @@
 
 mkdir -p $OUT || exit 1
 
-export DISK=$(mktemp -p $OUT -t XXXXXXXXXXXXXXXX.img)
+export DISK=$(mktemp --tmpdir=$OUT XXXXXXXXXXXXXXXX.img)
 export EFIVAR=$OUT/$(basename $DISK .img).fd
 
 truncate -s $size $DISK