Template variables for `cxx_genrule` do not work with `system_cxx_toolchain`

The [documentation for `cxx_genrule`](https://buck2.build/docs/api/rules/#cxx_genrule) lists a bunch of macros that are available in the context of the command such as `$(cxx)` or `$(cxxflags)`. These unkeyed `TemplatePlaceholderInfo()` variables are expected to come from `toolchains//:cxx` which is a dependency of `cxx_genrule`. These macros are very useful, obviously, for commands like `bindgen` that need access to the compiler flags.

However, it doesn't work OOTB with `system_cxx_toolchain`, because `system_cxx_toolchain` doesn't call `cxx_toolchain_infos` which sets up the appropriate `TemplatePlaceholderInfo()`. After some spelunking I found `cxx_toolchain`, a very handy rule, that does do that, and is a bit more appropriate for my use case where I download a toolchain and then point something to it.

Currently on Windows I use `system_cxx_toolchain` with a hack to avoid these template variables in that case, and though I'll probably migrate to `cxx_toolchain` on all platforms including Windows to mitigate this and make things more consistent, this should still probably be fixed OOTB for users.