S3Backup: fix compilation issue where MACHINE_LOCAL_STORE variable is…

… undefined

See #1414 (comment)

The variable is defined in src/lib/Hydra/Helper/Nix.pm

Error message without this patch:

```
hydra-evaluator[PID]: Couldn't require Hydra::Plugin::S3Backup : Global symbol "$MACHINE_LOCAL_STORE" requires explicit package name (did you forget to declare "my $MACHINE_LOCAL_STORE"?) at /nix/store/xxx-hydra-0-unstable-2024-09-24/libexec/hydra/lib/Hydra/Plugin/S3Backup.pm line 95.
hydra-evaluator[PID]: Compilation failed in require at /nix/store/xxx-hydra-perl-deps/lib/perl5/site_perl/5.38.2/Module/Runtime.pm line 314.
hydra-evaluator[PID]:  at /nix/store/xxx-hydra-perl-deps/lib/perl5/site_perl/5.38.2/Module/Pluggable.pm line 32.
```

src/lib/Hydra/Plugin/S3Backup.pm

@@ -14,6 +14,7 @@ use Nix::Config;
 use Nix::Store;
 use Hydra::Model::DB;
 use Hydra::Helper::CatalystUtils;
+use Hydra::Helper::Nix;
 
 sub isEnabled {
     my ($self) = @_;