feat: add pixi.init_cwd template variable for task inputs/outputs

[Premkumar-2004]

Description

Added pixi.init_cwd template variable so it can be used in task inputs/outputs for caching.
[tasks.mkdir_test] cmd = "mkdir -p {{ pixi.init_cwd }}/test" outputs = ["{{ pixi.init_cwd }}/test"]

Fixes #5475

How Has This Been Tested?

Added unit test test_template_string_renders_init_cwd and ran existing task tests

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: claude

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.