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

t/scripts/hydra-create-user.t

@@ -2,6 +2,7 @@ use strict;
 use warnings;
 use Setup;
 use Test2::V0;
+use Hydra::Helper::Exec;
 
 my $ctx = test_context();
 my $db = $ctx->db();

t/scripts/hydra-init.t

@@ -2,6 +2,7 @@ use feature 'unicode_strings';
 use strict;
 use warnings;
 use Setup;
+use Hydra::Helper::Exec;
 
 my %ctx = test_init();
 

t/scripts/hydra-send-stats.t

@@ -2,6 +2,7 @@ use feature 'unicode_strings';
 use strict;
 use warnings;
 use Setup;
+use Hydra::Helper::Exec;
 
 my %ctx = test_init();
 

t/scripts/hydra-update-gc-roots/update-gc-roots.t

@@ -3,6 +3,7 @@ use strict;
 use warnings;
 use Setup;
 use Test2::V0;
+use Hydra::Helper::Exec;
 
 my $ctx = test_context();
 my $builds = $ctx->makeAndEvaluateJobset(