Add workflow running exported actions for polkadot-sdk compatibility

[elle-j]

Summary

Exercises the run-differential-tests action the same way polkadot-sdk does, but against this repo's working tree and polkadot-sdk's master branch. The goal is to surface changes that would break polkadot-sdk's CI running the same action, which would otherwise only be detected once polkadot-sdk bumps the revive-differential-tests commit it pins. (Forklift is used for polkadot-sdk caching.)

Note

For instance, the current failed compatibility checks show that the polkadot-sdk CI will break if it'd bump the pinned commit to the current state.

Those specific failures may be due to the same failures revive had to solve by passing a different concurrency-number-of-concurrent-tasks.

[elle-j]

@0xOmarA, fyi, the compatibility will still fail for polkadot-sdk even if they set concurrency-number-of-concurrent-tasks: 1000. The failures for that specific change can be seen in the CI checks from this commit.