ci: replace Cachix with local Nix caches

[krtk6160]

Summary

• vendor ci: replace Cachix with local Nix caches galoy-concourse-shared#26
• use persistent /nix-cache volumes for check-code, integration, and BATS
• remove the Cachix token and cache-name pipeline values

Dependency

Land GaloyMoney/galoy-concourse-shared#26 first so the vendored commit is on main.

Validation

• vendir sync completed against the pinned shared commit
• ytt renders the full Bria pipeline successfully
• rendered pipeline contains no Cachix credential or command references
• vendored shell scripts pass syntax checks

[krtk6160]

Superseded by #669, which stacks the non-vendor local Nix cache changes onto the Concourse-generated shared-files bump.