fix(py-ray): constrain to older bazel

I just hit https://github.com/ray-project/ray/issues/31504 when trying to build "ebrains" on a unconstrained spack (0.20.x) → bazel@6.1.1 is chosen but is too new for py-ray:

[2023-11-20T13:28:52.477Z]   ERROR: /tmp/spack/home/.cache/bazel/_bazel_spack/0bdc9ee34fb83e076ebde0193a2f2f10/external/bazel_tools/platforms/BUILD:89:6: in alias rule @bazel_tools//platforms:windows: Constraints from @bazel_tools//platforms have been removed. Please use constraints from @platforms repository embedded in Bazel, or preferably declare dependency on https://github.com/bazelbuild/platforms. See https://github.com/bazelbuild/bazel/issues/8622 for details.
[2023-11-20T13:28:52.477Z]   ERROR: /tmp/spack/home/.cache/bazel/_bazel_spack/0bdc9ee34fb83e076ebde0193a2f2f10/external/bazel_tools/platforms/BUILD:89:6: Analysis of target '@bazel_tools//platforms:windows' failed
[2023-11-20T13:28:52.477Z]   ERROR: /tmp/spack/tmp/spack-stage/spack-stage-py-ray-2.0.1-cay2f2cshkpqelgfc26fkpjncott6j7w/spack-src/cpp/BUILD.bazel:6:10: While resolving toolchains for target //cpp:libray_api.so: invalid registered toolchain '@bazel_skylib//toolchains/unittest:cmd_toolchain':