Fix README to use Hardhat config vars instead of .env file

[zent7x]

Summary

• Fixed inconsistent documentation in the Ganache section of README.md
• The README previously instructed users to set the mnemonic in a .env file, but this template uses Hardhat Configuration Variables
• Updated the instruction to use bunx hardhat vars set MNEMONIC instead, which is consistent with the rest of the documentation

Test plan

• Documentation-only change
• Verified this matches the approach documented in the "Pre Requisites" section