CI: call CPack via CMake

CI: call CPack via CMake

CPack is CMake’s “packaging driver,” exposed as `cpack.exe`. Yet if you
call `cpack.exe` in some CI environments, you will find the binary in your
`$PATH` is _not_ CMake’s packaging driver. This is what the logic in the Windows
setup script that is being removed in this commit was attempting to detect and
workaround or diagnose: a `cpack.exe` in your `$PATH` that is some unrelated and
unhelpful platform management executable.

But there is a simpler way. CMake’s docs¹ tell us:

  Instead of `cpack`, one may call `cmake --build . --target package` or
  `make package` or `ninja package`.

In this change, we drop the `cpack` and `$PATH` workarounds and just call
`cmake`, relying on it to know where its own packaging driver is.

¹ https://cmake.org/cmake/help/latest/module/CPack.html