fix: ensure variants are used on first parsing pass

[beckermr]

Description

This PR ensures that variant specs are used on the first parsing pass through the recipe.

Consider this block of yaml:

requirements:
  run:
    - python {{ python_min }}.*

Currently, conda-build does NOT insert a value for python_min on the first parsing pass. This leads to an invalid spec for the version:

requirements:
  run:
    - python .*

By inserting the variants once first, we get valid specs, e.g. for python_min=3.12

requirements:
  run:
    - python 3.12.*

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[codspeed-hq]

CodSpeed Performance Report

Merging #5528 will degrade performances by 10.97%

_{Comparing beckermr:fix-undef-jinja (4120b7f) with main (bd94b7f)}

Summary

❌ 1 regressions
✅ 4 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	beckermr:fix-undef-jinja	Change
❌	test_render_recipe	25.1 s	28.2 s	-10.97%

[beckermr]

@conda/builds-tools this PR is ready for review!

It fixes the first parsing round so that it uses variables instead of ignoring them. This is needed to fix edge cases in parsing.

I think the small performance regression is acceptable given this is a bug fix.

[beckermr]

@jakirkham this should fix some edge cases with cuda variables that needed defaults in recipes.