assorted: ensure mktemp places temporaries in /tmp

hosts/by-name/servo/services/nixos-prebuild.nix

@@ -6,7 +6,7 @@ lib.optionalAttrs false  # disabled until i can be sure it's not gonna OOM my se
     description = "build a nixos image with all updated deps";
     path = with pkgs; [ coreutils git nix ];
     script = ''
-      working=$(mktemp -d /tmp/nixos-prebuild.XXXXXX)
+      working=$(mktemp -d nixos-prebuild.XXXXXX --tmpdir)
       pushd "$working"
       git clone https://git.uninsane.org/colin/nix-files.git \
         && cd nix-files \

pkgs/by-name/opencellid/package.nix

@@ -42,7 +42,7 @@ stdenv.mkDerivation {
       runtimeInputs = [ curl git gzip ];
       text = ''
         set -x
-        pushd "$(mktemp -d opencellid.XXXXXXXX)"
+        pushd "$(mktemp -d opencellid.XXXXXXXX --tmpdir)"
 
         git clone git@git.uninsane.org:colin/opencellid-mirror.git
         cd opencellid-mirror