Do not use CMAKE_INSTALL_BINDIR in EkatSetNvccWrapper.cmake

[bartgol]

Describe the bug

The following output shows the problem better than I could.

CMake Warning (dev) at /home/projects/ppc64le/cmake/3.18.0/share/cmake-3.18/Modules/GNUInstallDirs.cmake:225 (message):
  Unable to determine default CMAKE_INSTALL_LIBDIR directory because no
  target architecture is known.  Please enable at least one language before
  including GNUInstallDirs.
Call Stack (most recent call first):
  /home/e3sm-jenkins/weaver/workspace/SCREAM_PullRequest_Autotester_Weaver/621/scream/externals/ekat/cmake/mpi/EkatSetNvccWrapper.cmake:23 (include)
  CMakeLists.txt:39 (EkatSetNvccWrapper)
This warning is for project developers.  Use -Wno-dev to suppress it. 

To Reproduce
Should be reproducible on any GPU build.

Expected behavior
We don't want warnings in our config output.

Possible solution
The bin dir is universally ${CMAKE_INSTALL_PREFIX}/bin, so we can probably hardcode that, and avoid including GNUInstallDirs in the script.