don't include derivation name in temporary build directories

[Mic92]

With the migration to /nix/var/nix/builds we now have failing builds when the derivation name is too long.
This change removes the derivation name from the temporary build to have a predictable prefix length:

Also see: NixOS/infra#764
for context.

Motivation

Context

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[Mic92]

I would like to backport this to nix 2.30/2.31, so that we can also upgrade hydra.

[xokdvium]

@Mic92, is this actually enough savings to make Darwin builds not fail? We can do a bit more drastic savings to err on the safe side. See Emily's patches to Lix for other options. I imagine we can also rename var/builds to var/b

[Mic92]

@Mic92, is this actually enough savings to make Darwin builds not fail? We can do a bit more drastic savings to err on the safe side. See Emily's patches to Lix for other options. I imagine we can also rename var/builds to var/b

I think it its. Names should be now shorter than previously with /tmp.

[Mic92]

Also just found out that lix is doing similar things now: https://git.lix.systems/lix-project/lix/commit/ce6dcf18d6e5b0a50bcc0059bd18867d8ac9f8da