msvc-cl is always called with /std:c++17

I want to compile using the C++20 standard. With the Linux GCC compiler, setting `--action_env=BAZEL_CXXOPTS="-std=c++20"` works fine (setting `--cxxopt=-std=c++20` works as well).

However, there are problems and unexpected behavior with Windows msvc-cl:

`--action_env=BAZEL_CXXOPTS="-std=c++20"` seems to be completely ignored. I noticed this issue in both Bazel versions 8 and 9.

Therefore, the only solution is to set `--cxxopt=-std:c++20`. This workaround is effective as long as `rules_cc` has **not** been used. This was possible under Bazel 8.

However, the following problem arises as soon as you enter the line `bazel_dep(name = "rules_cc", version = "0.2.16")` (the same applies to version 0.2.14) in `MODULE.bazel`. In this case, cl is called with both `/std:c++17` and `/std:c++20`. This results in the warning "cl : Command line warning D9025 : overriding '/std:c++17' with '/std:c++20'".

Ideally, `action_env` would work the same way on Windows as it does on Linux.
However, `rules_cc` should not pass `/std:c++17` for no reason, regardless of the user-set standard. It is also unclear why there is a difference in behavior with or without `rules_cc` (at least at Bazel 8). Or did I use something incorrectly?

(Note: If anyone wants to suggest using `/` instead of `-`, know that it doesn't matter; the behavior is the same.)