Using Rtools40v2 to test bindings on Windows

[Ilia-Kosenkov]

This fixes #53.
Rtools40v2 has been recently released.
It includes our hack to please rust compiler and resolves gcc linking issue.

libR-sys can now be safely tested using Rtools only (no dependence on msys2 is required for pre-computed bindings).

However, the setup-r action currently have a bug that does not allow installing other versions of Rtools40 even if the version string is explicitly specified (see my issue here r-lib/actions#289).
There is also a PR to make the new version default (r-lib/actions#287), which will not work until the issue is resolved.

I suggest to leave this PR as a draft for now. As soon as setup-r gets updated, we will remove the relevant code (marked with comments in the .yaml) and will finally have a working setup:
https://github.com/Ilia-Kosenkov/libR-sys/blob/a55515f4a999adfa4535aaed57112e3a233864ef/.github/workflows/test.yml#L215-L227

[Ilia-Kosenkov]

So Rtools40v2 was pushed to chocolatey and other places, and Windows images here got updated as well.
The Windows-specific job runs tests using only Rtools dependency and with no magic tricks, which (I hope) means that at least {rextendr} can work without additional dependencies.

Note that in this scenario, we use the default Rust toolchain for Windows, stable-x86_64-pc-windows-msvc.
It depends on Visual Studio Build Tools, which is basically a set of native compilers, linkers and various resources.
The stable-msvc toolchain is then used to cross-compile to pc-windows-gnu, using Rtools' mingw toolset.
In short, on Windows we now use default native compilers, default native Rust toolchain, and standard Rtools distribution, and no extra configuration (apart from PATH I guess) is needed.

[clauswilke]

Looks good to me.