Consider using full GH runner in CI

[Kailai-Wang]

Context

We are using self-hosted runner to build docker images in our CI so far, see:
#259
#393
#409

the main reason is self-hosted runner is much faster than GH-runner: ~20m vs 1h
But now self-hosted runner needs 40min as well to build the docker image (mostly because of polkadot update that introduces more compilation deps).

Maybe it's worth checking again how long the compilation on GH-runner would be and switch to it when possible. It will help concurrent CI runs.

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[Kailai-Wang]

sccache within Dockerfile only works with self-hosted runner.

Switching to gh-runner only makes sense unless we want to use cargo-chef + buildx local/gha cache