move use of input variables into env vars

[nickfyson]

This PR moves a number of uses of ${{ inputs.foo_bar }} into environment variables for the step in question.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[Copilot]

Pull Request Overview

This PR refactors GitHub Actions workflows by moving direct input variable usage into environment variables for improved readability and maintainability.

• In the release-branches action, inputs for "major_version" and "latest_tag" are now set as environment variables.
• In the prepare-test action, inputs for "version" and "use-all-platform-bundle" are similarly moved into environment variables and used in conditional checks.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.github/actions/release-branches/action.yml	Introduces env vars for MAJOR_VERSION and LATEST_TAG, and updates command args
.github/actions/prepare-test/action.yml	Adds env vars for VERSION and USE_ALL_PLATFORM_BUNDLE, and updates comparison logic