Consider adding `user_compile_flags` to cargo_build_script's `get_cc_compile_args_and_env()`

While investigating how to build rust `*-sys` crates under rules_rust with custom c/cxx/ld flags (via the `--copt|cxxopt|linkopt` bazel build options) I discovered that:

1. The rules_rust build script wrapper _does_ honor `--linkopt`s (settings are reflected in `LDFLAGS`)
2. But seemingly ignores `--copt` & `--cxxopt` (settings are *not* reflected in `CFLAGS`/`CXXFLAGS`)

This confused me for a couple hours, but I eventually discovered that seemingly bazel's cc_toolchain (?) does not reflect these in the normal flow of things, and requires one to explicitly pass `fragments.cpp.copts` + `fragments.cpp.cxxopts` to `cc_common.create_compile_variables()` for them to show up in `cc_common.get_memory_inefficient_command_line()` calls.

This is the patch I have for myself:

cargo/private: add user_compile_flags to honor copt/cxxopt

```diff
diff --git a/cargo/private/cargo_build_script.bzl b/cargo/private/cargo_build_script.bzl
index be08907e..f15d5810 100644
--- a/cargo/private/cargo_build_script.bzl
+++ b/cargo/private/cargo_build_script.bzl
@@ -119,7 +119,7 @@ https://github.com/bazelbuild/bazel/issues/15486
executable = True,
)

-def get_cc_compile_args_and_env(cc_toolchain, feature_configuration):
+def get_cc_compile_args_and_env(ctx, cc_toolchain, feature_configuration):
"""Gather cc environment variables from the given `cc_toolchain`

Args:
@@ -135,6 +135,7 @@ def get_cc_compile_args_and_env(cc_toolchain, feature_configuration):
compile_variables = cc_common.create_compile_variables(
feature_configuration = feature_configuration,
cc_toolchain = cc_toolchain,
+ user_compile_flags = ctx.fragments.cpp.copts + ctx.fragments.cpp.cxxopts,
)
cc_c_args = cc_common.get_memory_inefficient_command_line(
feature_configuration = feature_configuration,
@@ -386,7 +387,7 @@ def _cargo_build_script_impl(ctx):
env["LDFLAGS"] = " ".join(_pwd_flags(link_args))

# MSVC requires INCLUDE to be set
- cc_c_args, cc_cxx_args, cc_env = get_cc_compile_args_and_env(cc_toolchain, feature_configuration)
+ cc_c_args, cc_cxx_args, cc_env = get_cc_compile_args_and_env(ctx, cc_toolchain, feature_configuration)
include = cc_env.get("INCLUDE")
if include:
env["INCLUDE"] = include
```

---

This will have a knock on effect for anyone who's doing c/cpp and rust development in the same workspace / repo, so it may not be worth the disruption, but it strikes me as odd that rules_rust honors `--linkopt` (which is also a [cpp.fragment](https://bazel.build/rules/lib/fragments/cpp#linkopts)), but not the other two.

I'm looking for input from a maintainer about whether this is something that should be fixed, or if not; perhaps clarified in the docs somewhere?