Cap pixi-build-cmake at 0.3.11 and pixi below 0.71 (restores CPU-only builds)

[joshkamm]

Why

CPU-only builds are currently broken, silently.

pixi-build-cmake stopped forwarding the ambient build environment in 0.3.12 — 0.3.11 is the last version that still does. Verified across every version in between:

backend	MYFLAG=OFF visible to CMake?
0.3.8 – 0.3.11	yes
0.3.12 – 0.3.14	no

CMakeLists.txt reads the toggle via $ENV{USE_ACC} and defaults to ON, so on 0.3.12+ USE_ACC=OFF is ignored and a CPU-only request produces a GPU build that exits 0 with no warning.

Because pixi.lock does not record backend versions, which backend you get depends on when your machine last solved — so this reproduces for some people and not others on the identical commit.

What

• backend >=0.3.6,<=0.4.5 → >=0.3.6,<=0.3.11
• requires-pixi >=0.60.0 → >=0.60.0,<0.71

The pixi cap matters for the error message. pixi 0.71 is the first release to drop build-API v4, and without the cap you get a failure deep in the solver that never mentions pixi:

× could not initialize the build-backend
╰─▶ pixi-build-cmake >=0.3.6,<=0.4.5 cannot be installed:
    ├─ 0.4.5 → pixi-build-api-version >=6,<7 — no candidates found
    └─ 0.3.7 … 0.3.14 → api >=4,<5 — no candidates found

With it, pixi refuses before resolving and names the fix:

× this project requires pixi '>=0.60.0,<0.71', but you have pixi 0.80.0
help: install a version of pixi that satisfies '>=0.60.0,<0.71' with
      `pixi self-update --version <version>`

Verification

Full clean CPU build of the ZEST + SlaterGPU stack against this branch, on Perlmutter:

Instantiated backend pixi-build-cmake@0.3.11 (x2)
-- USE_ACC: OFF   (slatergpu)
-- USE_ACC: OFF   (zest)
BUILD_EXIT=0
8 passed, 9 skipped

Temporary

Both caps are stopgaps. They pin the lab off pixi 0.71+ deliberately, and should be lifted once the upstream issues are resolved — the ambient environment is not a supported configuration channel any more, so the real fix is a supported mechanism that can reach a source dependency. Upstream reports to follow; ZEST needs the matching cap (each package resolves its backend independently, so capping one layer alone leaves the other silently GPU).

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