fix(nix/runTestsuite): use s6-portable-utils

Same as 221698c603dcb318c609b4d21cb2a9fada44a14c

We had a bunch of instances of
https://github.com/NixOS/nix/issues/2176,
where nix would exit with a “killed by signal 9” error.

According to Eelco in that issue, this is perfectly normal behaviour
of course, and appears if the last command in a loop closes `stdout`
or `stdin`, then the builder will SIGKILL it immediately. This is of
course also a perfectly fine error message for that case.

It turns out that mainly GNU coreutils exhibit this behaviour …

Let’s see if using a more sane tool suite fixes that.

Change-Id: Iaf9e542952ca36c02208a3f067f575ba978272b4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2663
Reviewed-by: Profpatsch <mail@profpatsch.de>
Tested-by: BuildkiteCI

nix/runTestsuite/default.nix

@@ -37,7 +37,8 @@ let
     bool
     ;
 
-  bins = depot.nix.getBins pkgs.coreutils [ "printf" "touch" ];
+  bins = depot.nix.getBins pkgs.coreutils [ "printf" ]
+      // depot.nix.getBins pkgs.s6-portable-utils [ "s6-touch" ];
 
   # Returns true if the given expression throws when `deepSeq`-ed
   throws = expr:
@@ -161,7 +162,7 @@ let
         then depot.nix.runExecline.local "testsuite-${name}-successful" {} [
           "importas" "out" "out"
           "if" [ bins.printf "%s\n" "testsuite ${name} successful!" ]
-          bins.touch "$out"
+          bins.s6-touch "$out"
         ]
         else depot.nix.runExecline.local "testsuite-${name}-failed" {} [
           "importas" "out" "out"