getdeps: honor INSTALL_DIR correctly in the generated run_cmake.py script
getdeps: honor INSTALL_DIR correctly in the generated run_cmake.py script

Summary:
This cleans up how the `CMAKE_ENV` and `CMAKE_DEFINE_ARGS` variables are
written in the generated `run_cmake.py` script that we emit for CMake-based
projects.

We now emit each entry in these variables on separate lines, just to improve
readability.  (Both of these variables tend to have a number of entries and
are very long if emitted on a single line.)

This also replaces the `-DCMAKE_INSTALL_PREFIX` entry in `CMAKE_DEFINE_ARGS`
to have it correctly honor the `INSTALL_DIR` variable defined in
`run_cmake.py`.  This makes `run_cmake.py` still do the right thing if someone
manually edits it to change the `INSTALL_DIR` value.

Reviewed By: wez

Differential Revision: D16778006

fbshipit-source-id: fee5d25748b87b5d9c57ee2edf8de5e586e872ee

rsocket/rsocket-cpp,rsocket/rsocket-cpp,rsocket/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,rsocket/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,ReactiveSocket/reactivesocket-cpp