nix: put tmpdirs directly into /tmp to fix long unix socket paths

I'm not sure whether that's Lix specific, but when I enter nix-shell, I
get quite a long TMPDIR, which results in all postgrest-with-pg-XX
scripts failing, because the unix socket path is too long. If TMPDIR was
not set, mktemp would fall back to /tmp, so we can just do that in any
case, which guarantees a short path in every case.

nix/overlays/checked-shell-script/checked-shell-script.nix

@@ -104,7 +104,7 @@ let
         ''
 
         + lib.optionalString withTmpDir ''
-          tmpdir="$(${coreutils}/bin/mktemp -d --tmpdir ${name}-XXX)"
+          tmpdir="$(${coreutils}/bin/mktemp -d --tmpdir=/tmp ${name}-XXX)"
 
           # we keep the tmpdir when an error occurs for debugging
           trap 'echo Temporary directory kept at: $tmpdir' ERR