ci: bump pixi to v0.73.0 to fix per-run Cython rebuild (#2138) (#2396)

Pixi <0.71.0 re-ran the editable source build on every `pixi run`,
recompiling all ~29 Cython extensions in cuda_bindings (~27s) even when
nothing changed, because setuptools' editable_wheel uses fresh temp
build dirs so build_ext always sees the .so as older than its source.

Pixi 0.71.0 fixed this upstream with a content-addressed source-build
cache (prefix-dev/pixi#6285) plus separate Cython input tracking
(prefix-dev/pixi#6123). Locally verified: under 0.66 the 2nd no-op run
rebuilds 29 extensions with a fresh /tmp/*.build-temp; under 0.73 it
rebuilds nothing and returns instantly.

The fix also bumps the pixi.lock format from v6 to v7 (unreadable by
pixi 0.66), so this requires all contributors to move to pixi >=0.71.
Regenerated all six committed lock files to v7 accordingly.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: rparolin

.github/workflows/ci-pixi-source-test.yml

@@ -52,7 +52,11 @@ defaults:
     shell: bash --noprofile --norc -xeuo pipefail {0}
 
 env:
-  PIXI_VERSION: "v0.66.0" # keep in sync with the version developers run locally
+  # keep in sync with the version developers run locally. Must be >=0.71.0:
+  # older pixi re-ran the editable source build on every `pixi run`, recompiling
+  # all Cython extensions (#2138). The fix (content-addressed source-build cache,
+  # prefix-dev/pixi#6285 + #6123) also bumps the pixi.lock format to v7.
+  PIXI_VERSION: "v0.73.0"
 
 jobs:
   # ── PR guard: CPU-only build + import + placement smoke ──