brew: bump-formula-pr fails when attempting to sign commit

• Confirmed this is a problem with running a brew command and not brew installing or the post-install behaviour of one or more formulae? If it’s a formulae-specific problem please file this issue at the relevant tap e.g. for Homebrew/homebrew-core https://github.com/Homebrew/homebrew-core/issues/new
• Ran brew update and retried your prior step?
• Ran brew doctor, fixed all issues and retried your prior step?
• Ran brew config and brew doctor and included their output with your issue?

$ brew doctor 
Your system is ready to brew.

brew-config

To help us debug your issue please explain:

• What you were trying to do (and why) attempting to bump a formula with bump-formula-pr
• What happened (include command output) Error when it attempting to git-commit.

Already up-to-date.
==> replace "https://github.com/nodenv/node-build/archive/v2.6.19.tar.gz" with "https://github.com/nodenv/node-build/archive/v2.6.20.tar.gz"
==> replace "ed4b5aa3e5d6c474f7e53e9a78b6080bb7e57497ff1d2c67ddbc2700d0c5c2d0" with "af89b9eb0086324a78674d31e7d74e750d30ac8565b27058431b1b854b068b9a"
M	Formula/node-build.rb
Switched to a new branch 'node-build-2.6.20'
error: gpg failed to sign the data
fatal: failed to write commit object
Error: Failure while executing: git commit --no-edit --verbose --message=node-build\ 2.6.20 -- /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/node-build.rb

• What you expected to happen expected it to commit and open a PR
• Step-by-step reproduction instructions (by running brew commands)

My git configuration has user.signingkey set and commit.gpgsign=true.

Failed on: HOMEBREW_DEVELOPER=true brew bump-formula-pr --url=https://github.com/nodenv/node-build/archive/v2.6.20.tar.gz --sha256=af89b9eb0086324a78674d31e7d74e750d30ac8565b27058431b1b854b068b9a node-build

The commit succeeded when executed manually (by just re-running the exact command that was echoed in the failure output: git commit --no-edit --verbose --message=node-build\ 2.6.20 -- /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/node-build.rb). So I’m presuming the issue is that the brew process is running under a different user so it doesn’t have access to gpg-agent, thus it’s unable to sign the commit. However, I’m not sure what the remedy should be here. (This isn’t the first issue I’ve encountered with homebrew respecting the user’s gitconfig, but then failing due to some configuration therein.)

Is there some configuration that other homebrew devs use? I assume I’m not the only one who has git configured to sign commits.