Create weekly ci check to prevent nightly CI failures in contrib PRs

[benalleng]

This is pretty much a direct copy of rust-bitcoin's version to update the rust nightly version on a weekly basis.

TODO

• The individual workflows still download the latest nightly version but this will override that.
• Remove the push job run

Pull Request Checklist

Please confirm the following before requesting review:

• [x] I have disclosed my use of AI in the body of this PR.
• [x] I have read CONTRIBUTING.md and rebased my branch to produce hygienic commits.

[benalleng]

I am obviously setting up my PAT secret incorrectly. @spacebear21 any insight on what this failure indicates? I couldn't really find anything on it.

We want this runner to use a non-standard GITHUB_TOKEN so that the created PR activates the workflows to check the new version.

https://github.com/benalleng/rust-payjoin/actions/runs/18572035102/job/52948080221

[coveralls]

Pull Request Test Coverage Report for Build 18572107330

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 83.737%

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Totals
Change from base Build 18413003453:	0.0%
Covered Lines:	8954
Relevant Lines:	10693

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💛 - Coveralls