fix: only auto-update main branch

[samuelmaddock]

I noticed build-tools pulling down branches which aren't relevant to most users. I've changed auto-update to only pull down main/master.

% e build
Checking for build-tools updates
Running "git pull --rebase --autostash" in /Users/samuelmaddock/.electron_build_tools
From https://github.com/electron/build-tools
   dad2c81..c92af31  debug/ci   -> origin/debug/ci
build-tools is up-to-date

[dsanders11]

@samuelmaddock, this repo requires signed commits for merging.

[ckerr]

@samuelmaddock like there are some tests failing with this PR.

At first glance, it looks like it's just that the tests are expecting a specific output & those expectations need to be updated with this new git log message. Could you confirm & update the tests?

[dsanders11]

Test failure was fixed by rebasing on main (brought in #543, which fixed the failure). Force pushed to get a signed commit so this can be merged.