Removes msys2 dependency

[Ilia-Kosenkov]

Closes #91.
I have verified this behavior twice:

• Using a VBox Windows 10 VM with VC++ Build Tools, Rust/Cargo, R-4.1.0 and Rtools40v2 (clean installation, no msys2 on the system)
• Using the same CI with reset cache, check the output here
  I used an example by @yutannihilation posted in msys2 is no longer needed on Windows? #91, where the path was forced to point only to R and cargo binaries and rtools.
  Under these conditions, the CI successfully executes devtools::test(), which means the Rust code is indeed compiled successfully.

This can be separately verified by any Windows user by purging msys2 (or removing it from PATH) and then running something like rextendr::rust_eval("5i32 + 10i32").