Newer GHC refuses to allow "-O" with interactive mode, so let's filter
that out. Furthermore, sometimes you don't have a clean tree exactly
when you need to look up something/update the tags, so let's filter
out the "-Werror" too.

And finally, we need to pass the actual exact flags (including
nocurl, parallel, etc.) that we use for building, so let's add those
too.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>