"GET returned 401 Unauthorized"
Sounds like Bazel 0.29.0 issue 9327 [1] (unrelated to proxy). Should
be fixed in 0.29.1 / 1.0.0rc2 (now in .bazelversion of stable-3.0).
Could you see if that works?
When using Bazelisk with a .bazelversion showing 0.29.0, you may want
to temporarily set USE_BAZEL_VERSION=0.29.1 as environment variable.
HTH
[1]:
https://github.com/bazelbuild/bazel/issues/9327