Fix CI Builds for Nix and Rust

[vpetersson]

This PR addresses several issues causing failures in the Nix and Rust CI workflows.

Context:

• The Nix build started failing due to a Cargo.lock file generated with a newer Rust version (v4 format) being incompatible with the older Rust toolchain (1.77.2) provided by the nixos-24.05 Nixpkgs channel.
• The Nix cache action (magic-nix-cache-action) was also deprecated and causing errors.
• After resolving the initial Nix issues by reverting Cargo.lock, subsequent Rust test failures and compilation errors occurred due to code incompatibilities with Rust 1.77.2 and platform-specific test behavior.
• The Rust workflow was using the abandoned actions-rs/toolchain action.

Changes:

• .github/workflows/nix.yml:
  • Replaced the deprecated DeterminateSystems/magic-nix-cache-action with the recommended DeterminateSystems/flakehub-cache-action.
  • Added necessary permissions block (id-token: write, contents: read) for the flakehub-cache-action.
  • Reverted Cargo.lock to the v3 format version compatible with Rust 1.77.2 from the nixos-24.05 channel (done manually, reflected in git history).
• .github/workflows/rust.yml:
  • Removed the abandoned actions-rs/toolchain action.
  • Implemented a matrix strategy to test against latest, 1.86.0 (stable), and 1.77.2 Rust versions.
  • Switched build and test steps to execute within Docker containers using the official rust images corresponding to the matrix versions.
  • Added caching using actions/cache@v4 for Cargo registry/git data and the ./target directory to improve performance.
  • Added Cargo.lock to workflow trigger paths.
• src/commands/edge_app/utils.rs:
  • Modified the failing test test_transform_edge_app_instance_path_to_instance_manifest_with_env_instance_override_should_return_overrided_manifest_path to compare only the filenames instead of full paths. This resolves failures caused by differing temporary directory paths between macOS and Linux build environments.
• src/commands/serde_utils.rs:
  • Replaced the use of Option::is_none_or (stabilized in Rust 1.86) with the equivalent Option::map_or(|s| s.is_empty()) logic to ensure compatibility with the older Rust 1.77.2 toolchain used in the Nix build.

Reasoning:

• Reverting Cargo.lock was chosen over overriding the Rust version in flake.nix to maintain stability and avoid using the nixos-unstable channel.
• Switching the Rust workflow to Docker provides explicit control over the toolchain version and replaces the unmaintained action, while caching minimizes performance impact.
• Code modifications ensure compatibility with the required older Rust version and fix platform-specific test fragility.