nixos/nixos-installer: use temporary directory on target filesystem

nix build should store it's temporary files on target filesystem.
This should fix 'No space left on device' on systems
with low amount of RAM when there is a need to build something
like Linux kernel

nixos/modules/installer/tools/nixos-install.sh

@@ -83,8 +83,11 @@ if [[ ! -e $NIXOS_CONFIG && -z $system ]]; then
 fi
 
 # A place to drop temporary stuff.
+tmpdir="$(mktemp -d -p $mountPoint)"
 trap "rm -rf $tmpdir" EXIT
-tmpdir="$(mktemp -d)"
+
+# store temporary files on target filesystem by default
+export TMPDIR=${TMPDIR:-$tmpdir}
 
 sub="auto?trusted=1"