Add Pixi workflow to verify Pyodide build works

[IvanIsCoding]

Follow up of #1447

This uses pixi to run the Pyodide build. This is still higly experimental. It is related to #1416 and #1420 in the sense that pixi is to conda what uv is to pypi.

The Github Action workflow I created does the following:

• Installs Python 3.12.7 from Conda via Pixi. Python needs to be pinned
• Installs Emscripten 3.1.58. This is also pinned. Newer versions are required to build with Python 3.13 but that is not on Conda at the moment.
• Installs Node.js
• Installs Rust 1.86. Generally we'd use nightly rust, but Conda does not offer that. So I had to use the RUSTC_BOOTSTRAP trick to enable nightly features on a compiler. Of course that is terrible, but the version is pinned so essentially it is like using a pinned nightly compiler
• Installs a pinned version of https://github.com/pyodide/pyodide-build
• The Pyodide CLI installs the tools to build for 0.27.7, which maps to Python 3.12
• Compiles the code with Rust and Emscripten

Tests are included in #1450

[coveralls]

Pull Request Test Coverage Report for Build 20642734608

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.179%

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Totals
Change from base Build 20629792891:	0.0%
Covered Lines:	18284
Relevant Lines:	19414

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💛 - Coveralls

[IvanIsCoding]

Emscripten 4.0.9 seems to be broken with binaryen, I'll need to investigate. It's a shame because I wanted to bump Pyodide such that we can test with the Pyodide shipped by uv

[IvanIsCoding]

Looks like uv might support a version compiled
with emscripten 3.1.58