[core] Avoid re-deploying existing contracts from GH action

[bgins]

With the changes in #114, we deploy contracts to DevNet when we make changes to main. Contracts are blindly re-deployed, which is fine in testing environments but to be avoided when real money is involved.

Let's add a check to avoid re-deploying contracts. One possible approach would be to compile the contracts on the previous commit and then re-compile on the current commit. This may be enough to prevent hardhat from re-deploying, but it needs more investigation.