flake: fix devshell on macOS

[danieldk]

cudart was added as a direct dependency, but it does not exist on macOS,
so make it conditional on being supported on the platform.

[github-actions]

Coverage report — kernels/

Measured on: Python 3.10 / Torch 2.13.0.
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: 87.2% — threshold: 80% — ✅

Per-file breakdown

Name	Stmts	Miss	Cover	Missing
src/kernels/__init__.py	14	0	100%
src/kernels/_system.py	6	1	83%	10
src/kernels/_versions.py	130	14	89%	53, 59-60, 63-64, 102, 165-170, 199, 219
src/kernels/archs.py	56	1	98%	94
src/kernels/backends.py	213	62	71%	42, 46, 50-53, 70, 92, 110, 119, 123, 127-129, 150, 159, 163, 167-169, 190, 201, 203, 210-213, 226, 230, 234-254, 262, 285-305
src/kernels/compat.py	9	1	89%	5
src/kernels/deps.py	70	1	99%	56
src/kernels/hf_hub.py	63	2	97%	21, 23
src/kernels/importer.py	44	5	89%	80, 84, 87, 101-102
src/kernels/install.py	21	7	67%	76-100
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	85	6	93%	90, 115, 191, 311, 338, 368
src/kernels/layer/globals.py	5	0	100%
src/kernels/layer/kernelize.py	80	8	90%	258, 293, 301-302, 308, 312, 328-330
src/kernels/layer/layer.py	215	14	93%	182, 229, 256, 390, 470-471, 492, 500, 511, 540, 544, 557, 610, 640
src/kernels/layer/mode.py	14	0	100%
src/kernels/layer/repos.py	144	42	71%	27, 33, 36-43, 63-64, 70, 73-76, 90, 94, 103-104, 110, 113-116, 123-124, 130, 133-136, 143-144, 150, 153-156, 163-164, 170, 173-176, 257
src/kernels/load.py	71	2	97%	338, 378
src/kernels/locking.py	89	64	28%	35-83, 91-98, 102-125, 137, 152-159, 165-175, 179-186
src/kernels/python_deps.py	58	6	90%	59-60, 64-65, 101, 104
src/kernels/resolver.py	156	2	99%	220, 226
src/kernels/status.py	50	2	96%	25, 79
src/kernels/validate.py	88	5	94%	9, 100, 167, 190-191
src/kernels/variants.py	278	19	93%	65, 96, 117, 147, 256-257, 299-302, 304, 388-394, 400-406, 437-443, 455-461
src/kernels/verify.py	127	6	95%	46, 202-204, 318-319
TOTAL	2243	288	87%

Updated by the Test kernels workflow on commit b53e09817912a220dc695320a6dd3f69f392e219.