feat: add support for LoongArch64 architecture

[xflcx1991]

Description

This enables pixi to work on LoongArch 64-bit Linux systems, with proper PyPI package resolution using the standard 'loongarch64' identifier.

This is a follow-up to #4163

Fixes #5212

How Has This Been Tested?

1. cargo test -p pixi_manifest toml::platform PASS

pixi on feature/issue-#5212-add-fully-support-for-loongarch64-architecture [?] via 🐍 v3.14.2 via 🦀 v1.90.0 via 🧚 v0.62.2 
❯  cargo test -p pixi_manifest toml::platform
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.41s
     Running unittests src/lib.rs (target/debug/deps/pixi_manifest-c9a631246a0ba017)

running 1 test
test toml::platform::test::test_deserialize ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 264 filtered out; finished in 0.07s

3. build and test on loongarch64 machine

(ssh)root@YuanBao [ workspace_python ] # pixi --version
pixi 0.62.2
(ssh)root@YuanBao [ workspace_python ] # pixi init
✔ Created /buildroots/xffish/workspace_python/pixi.toml
(ssh)root@YuanBao [ workspace_python ] # cat pixi.toml 
[workspace]
channels = ["conda-forge"]
name = "workspace_python"
platforms = ["linux-loongarch64"]
version = "0.1.0"

[tasks]

[dependencies]
(ssh)root@YuanBao [ workspace_python ] # uname -a
Linux YuanBao 6.17.7-aosc-main-16k #1 SMP PREEMPT_DYNAMIC Mon Dec  1 14:32:31 UTC 2025 loongarch64 GNU/Linux

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.

[xflcx1991]

Hey @ruben-arts 👋,

Could you take a look at this PR when you have time?

Let me know if you have any questions or suggestions! Thanks!

[xflcx1991]

Hey @ruben-arts 👋,

Could you take a look at this PR when you have time?

Let me know if you have any questions or suggestions! Thanks!

pixi info and pixi add rust still have problem, so I'm putting off the review for now.

[baszalmstra]

I think rust is just not available for linux-loong64 yet on conda-forge, in fact I dont think there is any package available for that platform yet.

As for pixi info, if you are running this on a linux-loong64 machine, than that also looks correct no?

[ruben-arts]

Wait, what is the right format. I don't think there should be a difference between the toml and the actuall platform.

[ruben-arts]

It should be the same name as the subdir of the channel has. Assuming rattler has the correct string it should be linux-loong64.

[baszalmstra]

I think the name in rattler is wrong, I opened a PR: conda/rattler#1957