Different gas used in hardhat network with fork, without fork, and rinkeby

[fvictorio]

This is a simplified version of #990, but it's different enough to merit its own issue. Maybe fixing this (assuming there's something to be fixed) fixes that one too, but maybe not.

Reproduction steps

git clone https://github.com/fvictorio/hardhat-gas-used-bug
cd hardhat-gas-used-bug
export FORK_URL="https://mainnet.infura.io/v3/<project-id>"
export RINKEBY_URL="https://rinkeby.infura.io/v3/<project-id>"
export RINKEBY_PRIVATE_KEY="<rinkeby-account-private-key>"

npx hardhat run script.js # 43317
FORK=true npx hardhat run script.js # 43086
npx hardhat run script.js --network rinkeby # 58317

The three runs give a different gas used. To the best of my knowledge, the contracts don't use any on-chain state, so that shouldn't be the cause. But maybe I'm missing something here.