Avoiding rebuilds when updating keyring

[Ian Eure]

Hi Guixers,

I just hit the following irksome case again.  I cherry-picked a PR 
onto master, ran `make', built the package etc.  When pushing the 
commit, authentication failed.  This usually happens when master 
has a commit from a new contributor, but I haven’t pulled the 
keyring branch.

So I check out the keyring branch and pull it, then switch back to 
master.  This process invalidates all the .go files I just built, 
requiring another lenghy `make'.  Guix takes a shockingly long 
time to compile, even on powerful hardware.  I’m typing this from 
an i7-12700T with 12 physical cores (20 virtual with 
hyperthreading), and it takes around 13 minutes to build with the 
fans blasting the whole time.  It’s much worse on a laptop, where 
it can take 20+ minutes.

Anyone have a better way to do this?  `git pull' doesn’t seem to 
let you pull to anything other than the checked-out branch.  I 
guess I could edit .git/refs/heads/keyring by hand, but it sure 
seems like there should be a cleaner way to accomplish this.

 -- Ian