nixos/stage-1: use `readlink -e` in builder

The builder has this convoluted `while` loop which just replicates
`readlink -e`. I'm sure there was a reason at one point, because the
loop has been there since time immemorial. It kept getting copied
around, I suspect because nobody bothered to understand what it actually
did.

Incidentally, this fixes #17513, but I have no idea why.