ci: replace toolchain

[realeinherjar]

Description

This replaces actions-rs/toolchain for dtolnay/rust-toolchain in all CI runs.

Notes to the reviewers

Following the discussion in rust-bitcoin/rust-bitcoin#2113,
actions-rs/toolchain is an "Unofficial GitHub Actions for Rust programming language" which has only one person in the organization, @svartalf, which appears to be MIA for a long time.
People are asking in vain for these fixes, check actions-rs/toolchain#221 (almost a year long).

There are a bunch of warnings in CI, e.g. https://github.com/bitcoindevkit/bdk/actions/runs/6443147276/job/17499339102#step:6:71.

dtolnay/rust-toolchain is by a well-established Rust developer, and it is well-maintaned (and has more stars than actions-rs/toolchain.

Also rust-bitcoin already replaced actions-rs/toolchain with dtolnay/rust-toolchain in most every place

Changelog notice

Replaced actions-rs/toolchain with dtolnay/rust-toolchain in CI.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

[realeinherjar]

Yaml is complicated, things like 1.60.0 will be rounded to 1.6.
As a best practice use strings.
Also check YAML Hell

[realeinherjar]

Suggested change

	- uses: dtolnay/rust-toolchain@stable
	with:
	# we pin clippy instead of using "stable" so that our CI doesn't break
	# at each new cargo release
	toolchain: "1.67.0"
	# we pin clippy instead of using "stable" so that our CI doesn't break
	# at each new cargo release
	- uses: dtolnay/[email protected]

This could be simplified to this if desired

[LLFourn]

ConceptACK

[notmandatory]

If the team is generally on board with switching CI to a Nix with #1165 do we still need this PR?

[realeinherjar]

No, this is supersed by #1165.