Default value for extra_flags_per_feature CC configuration is invalid

[Silic0nS0ldier]

Describe the bug
Bazel is expecting a dict ({}) but is given list ([]).

To Reproduce
Use a Nix CC toolchain in Bazel 7.3.0 or 8.0.0-pre.20240807.1.

Expected behavior
Build passes.

Environment

• OS name + version: GNU/Linux (Ubuntu 22.04)
• Version of the code: a11818b

Additional context
Issue manifests as

RROR: /[...]/external/local_config_cc_nix_x86_64-linux/BUILD.bazel:92:20: @@local_config_cc_nix_x86_64-linux//:local: expected value of type 'dict(string, list(string))' for attribute 'extra_flags_per_feature' in 'cc_toolchain_config' rule, but got [] (list)
ERROR: Evaluation of query "@local_config_cc_nix_x86_64-linux//:local" failed

Issue noted on #574, new issue raised as I cannot open the closed issue (and technically there is a value now).

[aherrmann]

Will be addressed by #576