Avoid modifying frozen list

[linzhp]

repository_ctx.attr.exec_platforms and its values are all frozen. We cannot remove host platforms from them. So when we have this in WORKSPACE:

RBE_PLATFORMS = {
    "linux": ["amd64", "arm64"],
}

zig_toolchains(
  exec_platforms = RBE_PLATFORMS
)

Builds would fail on Linux because:

        File "/home/user/.cache/bazel/_bazel_zplin/391a7464ab75ac3a8cb620cc52c50a30/external/hermetic_cc_toolchain/toolchain/private/repositories.bzl", line 75, column 22, in _zig_sdk_repository_impl
                _archs.remove(_arch)
Error in remove: trying to mutate a frozen list value
ERROR: Target parsing failed due to unexpected exception: trying to mutate a frozen list value

This PR creates a regular dictionary out of the frozen one so we can remove the host platforms from exec_platforms

@birunts can you review?

[birunts]

Oh, good spot! Thanks 🙂

LGTM!

[albertocavalcante]

@linzhp, can a 4.0.1 be released with this fix? Thanks!

[linzhp]

released