captureStdoutStderr*: move to Hydra::Helper::Exec which helps avoid some environment variable fixation problems

t/queue-runner/build-locally-with-substitutable-path.t

@@ -3,10 +3,14 @@ use warnings;
 use Setup;
 use Data::Dumper;
 use Test2::V0;
+use Hydra::Helper::Exec;
+
 my $ctx = test_context(
     use_external_destination_store => 1
 );
 
+require Hydra::Helper::Nix;
+
 # This test is regarding https://github.com/NixOS/hydra/pull/1126
 #
 # A hydra instance was regularly failing to build derivations with: