This PR resolves the following problem. I'm using gpgsign for signing my commits. When I run ./script/lint.sh and get the error:

❯ ./script/lint.sh
linting .
0 issues.
linting example
...
linting tools/structfield
/Users/alexandear/src/github.com/google/go-github/tools/structfield
0 issues.
validating generated files
error: gpg failed to sign the data:
gpg: skipped "bot <bot@localhost>": No secret key
[GNUPG:] INV_SGNR 9 bot <bot@localhost>
[GNUPG:] FAILURE sign 17
gpg: signing failed: No secret key

fatal: failed to write commit object
failed validating generated files

This happens when ./script/lint.sh calls ./script/generate.sh with the --check argument. Therefore, I decided to explicitly disable commit.gpgsign to resolve the issue for all users of gpgsign.

P.S. I think ./script/generate.sh is overcomplicated and can be simplified. But this is another story and can be discussed in issue #3983.