When people reach out to the git repository they probably want to use
hydra from the same source.
This also removes the need for an overlay with simpler and more
performant direct use of the nixpkgs passed in. Before it was
re-importing nixpkgs.
test
There is an overlay for the `hydra` name, but `hydra_unstable` was used, which can refer to the nixpkgs package and lead to and outdated hydra version and requires configuring the correct package attribute downstream.
In my system logs I see this every time a new eval starts:
```
hydra-evaluator[PID]: hint: Using 'master' as the name for the initial branch. This default branch name
hydra-evaluator[PID]: hint: is subject to change. To configure the initial branch name to use in all
hydra-evaluator[PID]: hint: of your new repositories, which will suppress this warning, call:
hydra-evaluator[PID]: hint:
hydra-evaluator[PID]: hint: git config --global init.defaultBranch <name>
hydra-evaluator[PID]: hint:
hydra-evaluator[PID]: hint: Names commonly chosen instead of 'master' are 'main', 'trunk' and
hydra-evaluator[PID]: hint: 'development'. The just-created branch can be renamed via this command:
hydra-evaluator[PID]: hint:
hydra-evaluator[PID]: hint: git branch -m <name>
```
This ensures this hint is not logged anymore and unclutters the syslog.
I presume it does not really matter what name is chosen for the branch.
See https://github.com/NixOS/hydra/pull/1414#issuecomment-2412350929
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.
```
These are required for the `signString` and `readFile` subroutines used when signing NARs.
(cherry picked from commit b94a7b6d5c56362af9ea85d944f8454d861ec001)
