example how to use source maps

Hello,

i'm trying to use the interactive debugger in hevm. Contracts are compiled with `truffle compile`. Unfortunately I can not figure out how to set the source maps. I have tried like this

```
export ETH_RPC_URL=http://localhost:8545
export TXHASH=0x219cb5cca5708daa6c485c0688be6bbe6933cdea5fc802d3654c743452bbddf3
hevm exec --caller $(seth tx $TXHASH from) --address $(seth tx $TXHASH to) --calldata $(seth tx $TXHASH input) --rpc $ETH_RPC_URL --block $(($(seth tx $TXHASH blockNumber)-1)) --gas $(seth tx $TXHASH gas) --debug --json-file ./build/contracts/Contract.json
```
I always get `` in the debugger. I also can't seem to find it in the documentation.
I'd appreciate an example how to compile the contracts so I can debug a tx on chain with source code mapping.

Thank you