feat: add environment variable support to cache key templates

[pepicrft]

I added support for customizing the key, but forgot the most important piece that we need, being able to reference an environment variable value. Our CI runner provider is now exposing an environment variable that indicates if the OS configuration changes, and we plan to use that one to invalidate the cache, otherwise we get errors because the cached dependencies are linking against an invalid / non-existent glibc version.

Important

I wrote the code with claude code and reviewed it afterwards

Summary (Claude-generated)

• Add support for {{env.VAR_NAME}} syntax in cache key templates to allow reading environment variable values
• Enables more flexible cache key customization based on CI/CD environment variables like branch names, deployment environments, or custom build identifiers
• Maintains backward compatibility with existing cache key templates

Examples

# Include branch name from environment
cache_key: 'mise-{{env.GITHUB_REF_NAME}}-{{platform}}-{{file_hash}}'

# Use custom deployment environment
cache_key: 'mise-{{env.DEPLOY_ENV}}-{{platform}}-{{file_hash}}'

# Conditional logic with environment variables
cache_key: '{{default}}{{#if env.CUSTOM_SUFFIX}}-{{env.CUSTOM_SUFFIX}}{{/if}}'

Changes

• Modified processCacheKeyTemplate() in src/index.ts to include process.env in template data
• Updated action.yml documentation to include the new {{env.VAR_NAME}} syntax
• All existing functionality remains unchanged

Test plan

• Build and package successfully with npm run all
• Linting and formatting pass
• Manual testing with environment variables in cache key templates
• Verify backward compatibility with existing cache key configurations