x/build/cmd/gopherbot: should not consider backport requests inside backport issues

[dmitshur]

A backport issue should never itself be backported, so gopherbot should reject requests to backport inside such issues.

See #32261 (comment) where it happened by accident and caused confusing backport-backport issues to be created.

Also happened in #34881 (comment).

[gopherbot]

Change https://golang.org/cl/185897 mentions this issue: x/build/cmd/gopherbot: do not consider nested backport requests