py-jaxlib: Add explicit variant `cuda` (but disable)

Disabling py-jaxlib's cuda variant (inherited from CudaPackage) doesn't seem to be picked up when not defining the variant in the package itself.

Upstream they moved the variant from py-jax (which is pure Python) to py-jaxlib, cf. upstream PR 35460.

(CC: @dmolina1)

packages/py-jaxlib/package.py

@@ -21,6 +21,8 @@ class PyJaxlib(PythonPackage, CudaPackage):
     version("0.3.25", sha256="73ebc7868631cd9d520385557bbd7f08762d748a5a6a1bebef0f3b8d7ba748ef")
     version("0.1.74", sha256="bbc78c7a4927012dcb1b7cd135c7521f782d7dad516a2401b56d3190f81afe35")
 
+    variant("cuda", default=True, description="Build with CUDA")
+
     depends_on("python@3.7:", type=("build", "run"))
     depends_on("py-setuptools", type="build")
     depends_on("py-numpy@1.18:", type=("build", "run"))