chore: Turn off commit with gpgsign in script/generate.sh

[alexandear]

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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.52%. Comparing base (5bdc954) to head (ee6d198).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3982   +/-   ##
=======================================
  Coverage   93.52%   93.52%           
=======================================
  Files         207      207           
  Lines       17569    17569           
=======================================
  Hits        16432    16432           
  Misses        937      937           
  Partials      200      200           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[gmlewis]

Thank you, @alexandear!
LGTM.
Merging.