x/build/cmd/gopherbot: backport issues should retain topic labels of original

[bcmills]

GopherBot created #27498 from a source issue that had the labels “Documentation” and “modules”, but did not carry over those labels.

It makes sense for GopherBot to strip out status labels (such as NeedsFix), but I think it should retain the others.

(CC: @FiloSottile)