CI: do concurrent builds using Nix for XPU and Metal

[danieldk]

This was already done previously for CUDA/ROCm.

[github-actions]

Coverage report — kernels/

Measured on: Python 3.10 / Torch 2.12.0.
Other CI configurations are not included in this number.
Hardware-gated code paths (ROCm/XPU/NPU/Darwin/Windows) are excluded or unreachable on the Linux+CUDA runner.

Total coverage: 83.4% — threshold: 80% — ✅

Per-file breakdown

Name	Stmts	Miss	Cover	Missing
src/kernels/__init__.py	10	0	100%
src/kernels/_system.py	6	1	83%	10
src/kernels/_versions.py	63	7	89%	46, 49, 52-53, 56-57, 100
src/kernels/backends.py	194	55	72%	40, 44, 48-51, 68, 90, 108, 117, 121, 125-127, 148, 170, 181, 188-191, 201, 205-225, 233, 256-276
src/kernels/compat.py	8	1	88%	5
src/kernels/deps.py	54	4	93%	58-59, 79, 95
src/kernels/layer/__init__.py	6	0	100%
src/kernels/layer/_interval_tree.py	103	4	96%	23, 52, 147, 150
src/kernels/layer/device.py	48	14	71%	42, 47-49, 91, 96-98, 101, 149, 152, 155-157
src/kernels/layer/func.py	91	7	92%	72, 100, 154, 257, 263, 272, 290
src/kernels/layer/globals.py	5	0	100%
src/kernels/layer/kernelize.py	73	8	89%	255, 273, 281-282, 288, 292, 308-310
src/kernels/layer/layer.py	174	15	91%	156, 199, 205, 214, 306-307, 319, 328, 336, 347, 376, 380, 393, 446, 476
src/kernels/layer/mode.py	14	0	100%
src/kernels/layer/repos.py	130	34	74%	27, 33, 36-41, 61-62, 68, 71-74, 88, 92, 101-102, 108, 111-114, 121-122, 128, 131-134, 141-142, 148, 151-154, 235
src/kernels/lockfile.py	69	44	36%	37-98, 102-125
src/kernels/status.py	49	2	96%	23, 81
src/kernels/utils.py	287	59	79%	59, 71-75, 81-82, 207, 211, 214, 276, 301-302, 340, 369, 374, 408, 585-590, 616, 619, 621, 627, 640-641, 662-669, 673-680, 688, 692-702, 706-713, 751, 755, 774, 776
src/kernels/variants.py	262	19	93%	56, 87, 108, 138, 247-248, 289, 291, 371-378, 384-390, 421-427, 439-445, 534-536
TOTAL	1646	274	83%

Updated by the Test kernels workflow on commit 5085b2b443ea15050db626811eff5601bcb68f53.