ci: unify nightly versions in Cargo.toml

[satsfy]

Follow up from #254 (comment)

After the rbmt CI migration, most jobs were already consuming nightly from [package.metadata.rbmt.toolchains] in Cargo.toml, but the weekly updater still wrote nightly-version. This change makes Cargo.toml the single nightly source of truth across the updater and CI jobs.

Changes:

• update weekly nightly cron to use cargo rbmt toolchains --update-nightly. Replicates rust-bitcoin CI closely.
• nightly-version removed.
• migrate Embedded CI job to read nightly from Cargo.toml instead of nightly-version.

Validated in personal fork CI:

• The CI run (didn't create the PR because it wasn't allowed): https://github.com/satsfy/rust-bech32/actions/runs/24799694669/job/72578713007
• PR produced: https://github.com/satsfy/rust-bech32/compare/master...satsfy:rust-bech32:create-pull-request/weekly-nightly-update?expand=1

[apoelstra]

Can you just copy the files from rust-bitcoin so that I can diff them rather than re-reviewing? You might need to change the token but that should be it.

[satsfy]

It now mirrors rust-bitcoin's version in everything but 1. absent eval and 2. the repo specific token.

Validated again with this CI run creating this branch in my local CI.

Side question: Why don't we have a central CI repo for reusable workflows and shared actions (maybe just import from rust-bitcoin directly), and keep a tiny workflow file in each repo that only calls into that central logic?

[nyonson]

Side question: Why don't we have a central CI repo for reusable workflows and shared actions (maybe just import from rust-bitcoin directly), and keep a tiny workflow file in each repo that only calls into that central logic?

I have been attempting that with the actions in rust-bitcoin-maintainer-tools, but been finding them to be more trouble than they are worth since rbmt contains the bulk of the logic. Might be easier for repos to just copy/pasta something like this into their own repo since it won't change much.

[apoelstra]

ACK b8a32c9; successfully ran local tests

[tcharding]

Since you already diff'ed I can't add anything by reviewing this. But FWIW concept ACK.