Variants order might change in package.py when doing partial builds/releases

[herronelou]

Describe the bug here.

Environment

• OS - Windows 11 / Rocky 9
• Rez version - 2.112.0
• Rez python version - Python 3.7.9

To Reproduce

• Create a package.py with multiple variants
• Build the variants using the --variants arg, and build index 1, then repeat with index 0
  Resulting built package has variants out of order, which is problematic when the commands refer to a variant by index.

Expected behavior
Ideally preserve order, but might require padding the variants list with something until the other variants are built.

Actual behavior
Variant index changes

Why are we trying to do this
We're having problems building packages with variants for multiple OSs, we have to build the linux variants on linux, and the windows variants on windows. The order in which we do so is currently affecting the final order in the package.