oeqa/selftest/gotoolchain: Fix temp file cleanup

The go tests leave readonly files and directories behind.
Fix this to allow cleanup.

[YOCTO #14575]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 5680e95d)
Signed-off-by: Steve Sakoman <steve@sakoman.com>