assembler/qcow2: Move temporary image out of /tmp

Directory /tmp is hosted on tmpfs. Therefore the image size could be
limited by memory size. By moving the image to /var/tmp we assure that
the file is hosted on disk allowing us to build bigger images or build
images on memory-constrained machines (e.g. CI).

assemblers/org.osbuild.qcow2

@@ -45,7 +45,7 @@ def main(tree, output_dir, options, loop_client):
     if size % 512 != 0:
         raise ValueError("`size` must be a multiple of sector size (512)")
 
-    image = f"/tmp/osbuild-image.raw"
+    image = f"/var/tmp/osbuild-image.raw"
     mountpoint = f"/tmp/osbuild-mnt"
 
     # Create an empty image file