ci: build and test with pixi instead of CVMFS

[olantwin]

Summary

• All dependencies needed by this repo (geomodel 6.27.0 and its transitive closure: sqlite, xerces-c, nlohmann_json, eigen, geant4) are now published to the https://prefix.dev/ship channel via ship-conda-recipes, so CI no longer needs CVMFS access.
• New pixi.toml + committed pixi.lock define the build environment (cxx-compiler, cmake, ninja, make, git, sqlite, geomodel) and three tasks: configure, build, test (the latter two depend on the former).
• The workflow is rewritten to run on GitHub-hosted ubuntu-latest with prefix-dev/setup-pixi@v0.9.6 and a single pixi run test step — dropping the self-hosted runner, the registry.cern.ch/ship/gha-runner container, the three CVMFS volume mounts, and the LCG_109 / GeoModel 6.24.0-local2 source lines.

Test plan

• Local: pixi install && pixi run test on a plain shell (no LCG/distrobox) — 160/160 build targets, 25/25 ctests pass in ~13 s.
• CI: confirm the new Build and Test job runs on ubuntu-latest, installs the pixi env from the lock file, builds, and exits 0.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 86783a0f-a019-4fdf-baf0-80b735f07bd0

📥 Commits

Reviewing files that changed from the base of the PR and between a5bac91 and c3cf850.

⛔ Files ignored due to path filters (1)

• pixi.lock is excluded by !**/*.lock

📒 Files selected for processing (2)

• .github/workflows/build-test.yml
• pixi.toml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci/use-pixi

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