bug#78194: `guix pull --list-generations' is extremely slow

[Ian Eure]

I’ve been frustrated by this before, but decided to measure 
things:

`guix home list-generations' takes 3.535s to list 28 items.
`guix system list-generations' takes 1.406s to list 35 items.
`guix pull --list-generations' takes 232.599s to list 41 items.
Why is `guix pull --list-generations' >100x slower than `guix 
system list-generations'?

Thanks,

 -- Ian