chore: use 1.83.0 for dev, 1.80.0 for building contract

[r-near]

The Problem

We've been stuck in a Rust versioning trap:

• Our dev dependencies require Rust ≥1.81.0
• The existing near-sandbox version is only compatible with contracts built with Rust 1.80.0
• Using 1.81.0/1.82.0 hit compiler bugs (rust-lang/rust#64490)
• Using 1.83.0 for everything caused CompilationError(PrepareError(Deserialization)) in tests

The Solution

This PR implements a dual-toolchain approach:

1. Use Rust 1.83.0 for development environment and running tests
2. Use Rust 1.80.0 specifically for building contract WASM files

Key changes:

• Modified CI to install both toolchains without forcing an override
• Set 1.83.0 as default while allowing rust-toolchain files to control contract building
• Fixed caching strategy for multi-toolchain setup

This matches what works on our local setups - we develop with newer Rust versions, but contracts are compiled with 1.80.0 via local rust-toolchain files.

Why This Works

NEAR sandbox expects a specific WASM binary format produced by Rust 1.80.0. When using newer Rust versions, subtle changes in serialization format cause deserialization errors.

The local rust-toolchain files in contract directories now correctly take precedence during contract compilation, ensuring compatibility with NEAR sandbox.

All tests now pass in CI with this configuration.