common,w32: Remove duplicated backslashes when setting the homedir.

* common/homedir.c (copy_dir_with_fixup) [W32]: Fold double
backslashes.

This is in general no problem but when we hash or compare the directory
to test whether tit is the standard home directory, we may use a
different socket file and thus a second instance of a daemon.

• GnuPG-bug-id: T6833