ci(common): remove toolchain.txt

[eudelins-zama]

Closes https://github.com/zama-ai/fhevm-internal/issues/805

[mergify]

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Pull request overview

This PR updates the golden Rust container image Docker build workflow to simplify its structure and add support for configurable Rust versions. The workflow is being modified to support new input parameters and use an updated version of the common Docker workflow template.

Key Changes

• Added rust-version input parameter to allow dynamic Rust version selection (default: "stable")
• Removed pull_request trigger and two jobs (check-changes and determine-tag) to simplify workflow
• Changed input parameter naming from snake_case to kebab-case and modified tag to be optional with empty default

Comments suppressed due to low confidence (1)

.github/workflows/golden-container-images-docker-build-rust.yml:54

• The ref parameter uses 'format' with 'github.event.inputs.tag' to create a git tag reference, but the tag input now has a default value of empty string and is not required. When the tag is empty, this will create an invalid reference 'refs/tags/' which could cause checkout failures. Consider adding a check to only set the ref when the tag is non-empty, or validate that the tag input is provided.

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