Update as per paritytech/polkadot#2522

Martin Pughrequested to merge
s3krit/srtool:mp-2522 into master

Finally got around to making the changes necessary due to Basti's PR https://github.com/paritytech/polkadot/pull/2522

I verified with Basti that we do need to build in the runtime/$RUNTIME dir, and can't just pass it with -p $PACKAGE. Bit annoying, since it means the WASM blob ends up in a different location. We probably want to also update the README example output.

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