Automatic Rust version updates

[ThetaSinner]

Workflow to update the Rust version every 6 weeks to the latest version. But try to set it so that we don't update right as the new version releases, maybe a couple of weeks after each release.

In Holochain:

• versions/weekly/flake.nix
• rust-toolchain.toml

In Holonix:

• flake.nix

Create a PR with the version bumped as one commit.
Attempt a Clippy fix with clippy --fix as a second commit. That can be reverted if it makes mistakes.

AC:

• Automated PRs on Holochain and Holonix
• Common GitHub Action extracted that can do rust-toolchain.toml bumps in any repo, which we can then re-use on Kitsune2 etc

[ThetaSinner]

In Holochain:

• versions/weekly/flake.nix
• rust-toolchain.toml

In Holonix:

• flake.nix

Create a PR with the version bumped as one commit.
Attempt a Clippy fix with clippy --fix as a second commit. That can be reverted if it makes mistakes.

AC:

• Automated PRs on Holochain and Holonix
• Common GitHub Action extracted that can do rust-toolchain.toml bumps in any repo, which we can then re-use on Kitsune2 etc