nix-build in tmpfs

I build a lot of nix packages locally. Until nix-2.30 release nix-build command triggered builds in /tmp directory by default. As it’s not a tmpfs by default I used to enable boot.tmp.useTmpfs = true; to force all of /tmp into tmpfs to get slightly faster builds. In nix-2.30 nix switched its default build directory to /nix/var/nix/builds : ... `build-dir` no longer defaults to `$TMPDIR` ... This…