Cannot add extra actions using variables

Currently `@rules_cc//cc/toolchains/actions:compile_actions` doesn't contain `@rules_cc//cc/toolchains/actions:objc_compile`. If I want to define a toolchain that supports `objc_compile`, I would expect to be able to create `cc_args` like this:

```bzl
cc_args(
name = "my_user_compile_args",
actions = [
"@rules_cc//cc/toolchains/actions:compile_actions",
"@rules_cc//cc/toolchains/actions:objc_compile",
],
args = ["{flags}"],
format = {
"flags": "@rules_cc//cc/toolchains/variables:user_compile_flags",
},
iterate_over = "@rules_cc//cc/toolchains/variables:user_compile_flags",
)
```

But when I do this it fails with:

```bzl
ERROR: /Users/ksmiley/dev/keith/bazel-cc-toolchain-example/toolchain/llvm/BUILD.bazel:276:8: in _cc_args rule //toolchain/llvm:user_compile_args2:
Traceback (most recent call last):
File "/private/var/tmp/_bazel_ksmiley/9ab617f34b5a1a9018f0a6157e69be65/external/rules_cc~/cc/toolchains/args.bzl", line 46, column 29, in _cc_args_impl
validate_nested_args(
File "/private/var/tmp/_bazel_ksmiley/9ab617f34b5a1a9018f0a6157e69be65/external/rules_cc~/cc/toolchains/impl/args_utils.bzl", line 63, column 28, in validate_nested_args
type = get_type(
File "/private/var/tmp/_bazel_ksmiley/9ab617f34b5a1a9018f0a6157e69be65/external/rules_cc~/cc/toolchains/impl/variables.bzl", line 132, column 21, in get_type
fail("The variable {var} is inaccessible from the action {action}. This is required because it is referenced in {nested_label}, which is included by {args_label}, which references that action".format(
Error in fail: The variable @@rules_cc~//cc/toolchains/variables:user_compile_flags is inaccessible from the action @@rules_cc~//cc/toolchains/actions:objc_compile. This is required because it is referenced in @@//toolchain/llvm:user_compile_args2, which is included by @@//toolchain/llvm:user_compile_args2, which references that action
```

If I remove the fail https://github.com/bazelbuild/rules_cc/blob/848d56aa290b17535797f56ceafca39ab64d577f/cc/toolchains/impl/variables.bzl#L127-L136 it does seem to work as I expect, but I assume that would have some unintended side effects eventually?

My first attempt at this was to override the default feature, which suffered from the same issue.

Start with cc/toolchains/impl/variables.bzl at the validation around lines 127-136 and reproduce the failure using the issue's cc_args example with objc_compile. Trace how variable accessibility is checked for nested actions; done means the requested extra action works without weakening the intended validation, with coverage added for this case if the repository has a suitable test entry point.

Written by the indexing model from the issue text.