Fix brew-bump.sh script

[jsjoeio]

It looks like the brew-bump.sh is not working as expected 🤔

./ci/steps/brew-bump.sh: line 19: upstream: command not found
fatal: cannot run /Users/runner/.git-askpass.sh: No such file or directory
fatal: could not read Password for 'https://cdr-oss@github.com': Device not configured

https://github.com/cdr/code-server/runs/3305506315?check_suite_focus=true#step:5:5676

We'll need to fix this before the next release.

Resources:
https://github.com/mislav/bump-homebrew-formula-action