feat(nix,holonix): use the rust-toolchain.toml for all rust packages

[steveej]

1. initialize rust-toolchain.toml into each of the rust projects' repositories with the currently used rust toolchain, e.g. 1.77.0.
2. use rust-overlay's fromRustupToolchainFile to assemble the toolchain
3. optional but encouraged: ensure holonix only contains a single desired (holochain's?) toolchain ([BUG] Entering a coreDev shell fetches a Rust compiler we don't need #3696)

open questions:

1. should holonix support reading from the end-users project's rust-toolchain.toml or whether use holochain's version?

[ThetaSinner]

Bullet 3. possibly overlaps with -> #3696

[github-actions]

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