fix: block explorer favicon

[bap2pecs]

Summary

https://linear.app/snapchain/issue/SNA-574/op-chain-deployment-fix-explorer-favicon

fix the favicon to display our own logo instead of blcokscout's

https://github.com/blockscout/frontend/blob/main/docs/ENVS.md#favicon said FAVICON_MASTER_URL uses NEXT_PUBLIC_NETWORK_ICON as the default value but it's still showing blockscout. manually setting FAVICON_MASTER_URL doesn't work either

root cause:

per https://github.com/blockscout/frontend/releases/tag/v1.36.0

blockscout/frontend#2319 - ⚠️ FAVICON_GENERATOR_API_KEY has been removed due to the migration to a Node.js library solution

Test Plan

run docker logs -n 100 -f frontend
before:

🌀 Generating favicons bundle...
🛑 Error: FAVICON_GENERATOR_API_KEY variable is not provided.
👎 Unable to generate favicons bundle.

after:

Generating favicons...
Favicons generated successfully!
⏳ Replacing default favicons with freshly generated pack...
👍 Favicons bundle successfully generated.

run make l2-explorer-restart to redeploy. see https://explorer.tohma.snapchain.dev/ to verify