cmd/update.sh: don't switch to master when updating to a tag

[Bo98]

Fixes issues seen with the homebrew/brew:latest image where it doesn't have the stable branch setup so it does a detached 4.1.22 -> master -> stable. This PR avoids that intermediate master change by making that pull happen in the background to the working state.

The issues with homebrew/brew:latest will be fixed anyway with 4.1.23 when the vendored gems become a part of the tag, but this change is still worthwhile to prevent that class of issues in the future as downgrades generally don't work well with git and vendored gems. The checkout is also unnecessary I/O as we checkout stable immediately afterwards.

I will also look into fixing the branches in the Dockerfile (i.e. run brew update somewhere), which will also avoid the issue.

[MikeMcQuaid]

Thanks @Bo98. For this, Homebrew/install and Homebrew/actions/setup-homebrew: we should probably be avoiding checking out master ever, really, even as an intermediate step.

[Bo98]

Yeah, this fixes brew update but Homebrew/install etc still do their own thing. Homebrew/install in particular is one that should probably change since it almost always results in downgrades.

[MikeMcQuaid]

Homebrew/install in particular is one that should probably change since it almost always results in downgrades.

Agreed 👍🏻