fix DeprecationWarning: invalid escape sequence \S

rc.xsh

@@ -111,7 +111,7 @@ def dpkg_welse(cmd):
 def dpkg_which(cmd):
     cmd_path = shutil.which(cmd)
     assert cmd_path, 'cmd {!r} not found'.format(cmd)
-    search_match = re.search('^(\S+): ', $(dpkg --search @(cmd_path)))
+    search_match = re.search(r'^(\S+): ', $(dpkg --search @(cmd_path)))
     assert search_match, f'{cmd_path!r} not installed via dpkg'
     return search_match.group(1), cmd_path