Refactor Companion Update Flow for robustness

[joao-paulo-parity]

See #252 for context. Unfortunately we don't yet have an integration test suite to be confident about putting those changes in production. Regardless, I thought I should still have it ready here in case there's interest in trying it out with the staging environments, but otherwise this can come along with the PR for #251.

Motivation

• No need for manual logging before running commands as the run_cmd commands already do it
• The cleanup checks are now done eagerly instead of the old cleanup routine at the end of the update
• Meticulously check for errors and communicate them in the merge request

Tested scenarios

• https://github.com/paritytech/polkadot-for-processbot-staging/pull/5
• https://github.com/paritytech/polkadot-for-processbot-staging/pull/6

Google Cloud logs are more readable now:

Run git ["clone", "-v", "https://x-access-token:${SECRET}@github.com/paritytech/polkadot-for-processbot-staging.git"] in the current directory

Run git ["remote", "get-url", "paritytech"] in ./polkadot-for-processbot-staging

Run git ["remote", "add", "paritytech", "https://x-access-token:${SECRET}@github.com/paritytech/polkadot-for-processbot-staging.git"] in ./polkadot-for-processbot-staging

Run git ["fetch", "paritytech", "test_merge_commit"] in ./polkadot-for-processbot-staging

Run git ["branch", "-D", "test_merge_commit"] in ./polkadot-for-processbot-staging

Run git ["checkout", "--track", "paritytech/test_merge_commit"] in ./polkadot-for-processbot-staging

Run git ["fetch", "origin", "test_merge_commit_owner"] in ./polkadot-for-processbot-staging

Run git ["merge", "origin/test_merge_commit_owner", "--no-ff", "--no-edit"] in ./polkadot-for-processbot-staging

Run cargo ["update", "-vp", "sp-io"] in ./polkadot-for-processbot-staging

Run git ["status", "--short"] in ./polkadot-for-processbot-staging

Run git ["commit", "-am", "update Substrate"] in ./polkadot-for-processbot-staging

Run git ["push", "paritytech", "test_merge_commit"] in ./polkadot-for-processbot-staging

Getting the head SHA after a companion update in paritytech/test_merge_commit

Run git ["rev-parse", "HEAD"] in ./polkadot-for-processbot-staging

Companion updated; waiting for checks on github.com/paritytech/polkadot-for-processbot-staging/pull/6

Closes #252

[joao-paulo-parity]

Overall the implementation is finished but I'd like to try to do some manual testing on https://github.com/paritytech/polkadot-for-processbot-staging. Moving to draft for the time being.