feat: add environment variable support to cache key templates (#250)

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
```yaml
# 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
- [x] Build and package successfully with `npm run all`
- [x] Linting and formatting pass
- [ ] Manual testing with environment variables in cache key templates
- [ ] Verify backward compatibility with existing cache key
configurations

Author: pepicrft

action.yml

@@ -50,7 +50,8 @@ inputs:
     description: |
       Override the complete cache key (ignores all other cache key options).
       Supports template variables: {{version}}, {{cache_key_prefix}}, {{platform}}, {{file_hash}}, 
-      {{mise_env}}, {{install_args_hash}}, {{default}} and conditional logic like {{#if version}}...{{/if}}
+      {{mise_env}}, {{install_args_hash}}, {{default}}, {{env.VAR_NAME}} for environment variables, 
+      and conditional logic like {{#if version}}...{{/if}}
   experimental:
     required: false
     default: "false"

dist/index.js

@@ -351,10 +351,11 @@ async function processCacheKeyTemplate(template: string): Promise<string> {
   const defaultTemplate = Handlebars.compile(DEFAULT_CACHE_KEY_TEMPLATE)
   const defaultCacheKey = defaultTemplate(baseTemplateData)
 
-  // Prepare final template data including the default cache key
+  // Prepare final template data including the default cache key and env variables
   const templateData = {
     ...baseTemplateData,
-    default: defaultCacheKey
+    default: defaultCacheKey,
+    env: process.env
   }
 
   // Compile and execute the user's template