ostbuild: Fix DB saving with /tmp and builddir in different volumes

This fixes the case where /tmp resides in one volume, and the build
directory in a different one: by storing the temporary file in the
same directory as the target one, we avoid the case in which os.rename()
would cross file system boundaries.

Reviewed-by: Colin Walters <walters@verbum.org>

src/ostbuild/pyostbuild/jsondb.py

@@ -79,7 +79,8 @@ class JsonDB(object):
 
         current_time = time.gmtime()
 
-        (fd, tmppath) = tempfile.mkstemp(suffix='.tmp', prefix='tmp-jsondb-')
+        (fd, tmppath) = tempfile.mkstemp(suffix='.tmp',
+                prefix='tmp-jsondb-', dir=self._dirpath)
         os.close(fd)
         f = open(tmppath, 'w')
         json.dump(obj, f, indent=4, sort_keys=True)