BLD: Switch CI tests to use pixi for environments

[ColmTalbot]

This is an attempt to remove our need to generate containers in scheduled jobs and have them occasionally just fail because of upstream changes and instead use pixi (conda+pypi+rust) with a lock file.
The lockfile can be modified on a schedule using pull requests so we know updating our test environment won't break things.

[Copilot]

Pull request overview

This PR updates the project’s CI and supporting test/docs tooling to use Pixi-managed environments (with a lockfile) instead of prebuilt container images, aiming to reduce flakiness from upstream container changes and make environment updates more controlled.

Changes:

• Replace container-based GitHub Actions workflows with prefix-dev/setup-pixi and Pixi environments (unit tests, array-backend tests, docs build, latest-dynesty).
• Add Pixi workspace configuration (pixi.toml) and remove container build infrastructure (Dockerfile, conda env template, scheduled container build workflow).
• Adjust tests to work with the new CI setup (ROQ data path discovery + CI ROQ download/cache; reduce JAX dependency in a prior test; increase signal-delay in sampler interrupt test).

Reviewed changes

Copilot reviewed 11 out of 13 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/integration/sampler_run_test.py	Increases SIGINT trigger delay to reduce interrupt-test flakiness in CI.
test/gw/source_test.py	Makes ROQ basis directory discovery configurable (incl. env var) rather than hardcoding /roq_basis.
test/core/prior/base_test.py	Removes dependency on jax.numpy for a namespace assertion in a warning test.
pixi.toml	Defines Pixi environments/features for CI, docs, GW deps, array-backends, and dynesty variants.
docs/Makefile	Runs Sphinx via Pixi documentation environment.
containers/environment.yml	Removed legacy conda environment template used for container builds.
containers/Dockerfile	Removed legacy CI container build (including ROQ data baked into image).
.gitignore	Ignores Pixi and venv working directories.
.github/workflows/unit-tests.yml	Switches unit tests / sampler tests / array-backend tests to Pixi; adds ROQ download+cache job.
.github/workflows/pages.yml	Switches docs/notebook execution to Pixi documentation environment.
.github/workflows/latest-dynesty.yml	Switches dynesty-on-master testing to Pixi environment.
.github/workflows/build-containers.yml	Removes scheduled container build/push pipeline.

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[lucascolley]

I was randomly browsing the project after noticing that you use array-api-extra, and saw this too — very cool! Feel free to drop me a ping if you ever have questions about using Pixi or array-api-extra :)