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.

packages.yaml

@@ -12,3 +12,5 @@ packages:
         variants: [~cuda~rocm~valgrind~mkldnn~mpi~gloo+tensorpipe~onnx_ml]
     py-jax:
         variants: [~cuda]
+    py-jaxlib:
+        variants: [~cuda]

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"))