Add Pixi workflow to verify Pyodide build works

[IvanIsCoding]

Follow up of #1447. After the parent PR is merged, the diff of this PR will be smaller. True diff: https://github.com/IvanIsCoding/rustworkx/compare/pyodide-rayon...IvanIsCoding:rustworkx:pixi-pyodide

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 0.27.6, which maps to Python 3.12
• Compiles the code with Rust and Emscripten

There are no tests at the moment but as long as we build, recipes like https://github.com/pyodide/pyodide-recipes/pull/90 should catch some obvious test failures.

[coveralls]

Pull Request Test Coverage Report for Build 18793296589

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 94.186%

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Totals
Change from base Build 18781234584:	0.0%
Covered Lines:	18272
Relevant Lines:	19400

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💛 - Coveralls