Add CARDANO_MAINNET_MIRROR to Nix shell

[ana-pantilie]

Description

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated. If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages. New section is never added with the code changes. (See RELEASING.md)
• Code is formatted with fourmolu (use scripts/fourmolize.sh)
• Cabal files are formatted (use scripts/cabal-format.sh)
• hie.yaml has been updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[lehins]

LGTM
Thanks

[lehins]

I'll merge this PR when we are ready.
FYI. We normally do not let merge commits into feature branches and ask to rebase on master instead. There is a feature in the UI for this as well "Update with rebase":

[ana-pantilie]

@lehins Sorry, thank you for fixing it! I'll make sure to remember this for future PRs.

[lehins]

@ana-pantilie No worries, you had no way of knowing. I only pointed it out for the future.
Thank you for submitting this PR.