Remove hardcoded MSRV in github actions

[zebreus]

Description

[Copilot]

Pull request overview

This PR removes the hardcoded MSRV value from GitHub Actions workflows and centralizes the Rust toolchain selection via a new composite action driven by the rust-toolchain file.

Changes:

• Removes the MSRV environment variable from multiple workflows and switches those jobs to use a new load_toolchain composite action.
• Updates CI workflows (test.yaml, build.yml, benchmark.yaml, documentation.yaml, cloudcompiler.yaml) to call dtolnay/rust-toolchain@stable with the toolchain loaded from rust-toolchain.
• Cleans up the MSRV integration test suite by deleting the CI workflow MSRV check, leaving Dockerfile and rust-toolchain synchronization tests intact.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/integration/cli/tests/msrv.rs	Removes the test that enforced MSRV values inside GitHub workflow files, consistent with moving workflows off a hardcoded MSRV.
.github/workflows/test.yaml	Replaces use of env.MSRV with the load_toolchain composite action across several jobs to derive the Rust toolchain from rust-toolchain.
.github/workflows/documentation.yaml	Removes the workflow-level MSRV env and uses load_toolchain to select the Rust toolchain for documentation builds.
.github/workflows/cloudcompiler.yaml	Drops the MSRV env var and uses load_toolchain to install the correct Rust toolchain before building cloudcompiler.wasm.
.github/workflows/build.yml	Updates the main build matrix to use load_toolchain instead of env.MSRV when installing Rust for multi-target builds.
.github/workflows/benchmark.yaml	Switches the benchmark job to use load_toolchain rather than env.MSRV for Rust installation.
.github/actions/load_toolchain/action.yml	Adds a new composite action that reads the toolchain from the rust-toolchain file and exposes it as an output for workflows to consume.

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