[DEVOPS-916] nix: Fix slow eval of cardano-sl-explorer-frontend

[rvl]

Description

yarn2nix needs to have a src argument to generate its nix expression (at evaluation time). It's not good if building src depends on building cardano-sl.

Linked issue

https://iohk.myjetbrains.com/youtrack/issue/DEVOPS-916

Type of change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• 🛠 New feature (non-breaking change which adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)
• 🏭 Refactoring that does not change existing functionality but does improve things like code readability, structure etc
• 🔨 New or improved tests for existing code
• ⛑ git-flow chore (backport, hotfix, etc)

Developer checklist

• I have read the style guide document, and my code follows the code style of this project.
• If my code deals with exceptions, it follows the guidelines.
• I have updated any documentation accordingly, if needed. Documentation changes can be reflected in opening a PR on cardanodocs.com, amending the inline Haddock comments, any relevant README file or one of the document listed in the docs directory.

Testing checklist

• I have added tests to cover my changes.
• All new and existing tests passed.

QA Steps

nix-instantiate -A cardano-sl-explorer-frontend
# (verify that cardano-sl is not built or fetched)
nix-build -A cardano-sl-explorer-frontend
ls -l ./result

[deepfire]

Most importantly this moves the cardano-explorer-hs2purs invocation (and therefore the dependency on cardano-explorer and cardano-sl) from the attribute defining the frontend source to the actual frontend build -- so it only gets evaluated when we build, not when we evaluate the attribute set.

[rvl]

Yes, exactly. Thanks.