Well in an attempt to make myself more informed and give a more complete answer I decided to deliberately break a build and run it with nix-build -K so I could show you it was under /tmp (I know this wouldn’t answer the installer question, but one thing at a time).
error: could not compile `XXX` (bin "XXX") due to 1 previous error; 4 warnings emitted
note: keeping build directory '/nix/var/nix/builds/nix-666327-2838463246/build'
So not /tmp.
~/src/work/xxx/xxx inline-pkgs +1 !1 ▼ impure 16:40:35
❯ ll /nix/var/nix/builds/nix-666327-2838463246/build
drwxr-xr-x - nixbld1 25 Sep 16:38 cargo-vendor-dir
.rw------- 8.0k nixbld1 25 Sep 16:38 env-vars
drwxr-xr-x - nixbld1 25 Sep 16:38 xxx
.rw------- 111 nixbld1 25 Sep 16:38 tmp.4I0df9OtRn
I am pretty confident tmp.4I0df9OtRn is the private builder tmp.
I had forgotten that 2.30 moves default build-dir because of reasons. So ignore everything I said about builds in tmpdir it’s all out of date, and it happens in /nix. Which doesn’t answer the installer question.
EDIT: having spent 30 seconds reading the installer, the call to nix-build has an undocumented --store (afaict) option that uses the mount point, so I would assume builds happen under /mnt/nix.