fix binary dirs for config files so elsa_testing works again

Our install testing currently fails due to

-- CPM: adding package Quickvec@ (master)
73-- Found CUDA: /usr/local/cuda (found version "10.0") 
74--     No dependencies added for quickvec, as it is not stand-alone
75-- CUDA and clang available - requirements fullfilled.
76CMake Warning at build/_deps/quickvec-src/cmake/ComputeCapabilityCUDA.cmake:47 (message):
77  Automatic GPU detection failed.  Defaulting to 3.0.  You can also set
78  CUDA_ARCH_TYPES manually.
79Call Stack (most recent call first):
80  build/_deps/quickvec-src/CMakeLists.txt:74 (CUDA_DETECT_INSTALLED_GPUS)
81--     quickvec testing is disabled
82-- Found OpenMP_CXX: -fopenmp=libomp (found version "3.1") 
83CMake Error at /usr/local/share/cmake-3.18/Modules/CMakeFindDependencyMacro.cmake:47 (find_package):
84  By not providing "Findquickvec.cmake" in CMAKE_MODULE_PATH this project has
85  asked CMake to find a package configuration file provided by "quickvec",
86  but CMake did not find one.
87  Could not find a package configuration file provided by "quickvec" with any
88  of the following names:
89    quickvecConfig.cmake
90    quickvec-config.cmake
91  Add the installation prefix of "quickvec" to CMAKE_PREFIX_PATH or set
92  "quickvec_DIR" to a directory containing one of the above files.  If
93  "quickvec" provides a separate development package or SDK, be sure it has
94  been installed.
95Call Stack (most recent call first):
96  /builds/IP/elsa/install-elsa/lib/cmake/elsa/elsaConfig.cmake:38 (find_dependency)
97  /usr/local/share/cmake-3.18/Modules/CMakeFindDependencyMacro.cmake:47 (find_package)
98  cmake/Findelsa.cmake:67 (find_dependency)
99  examples/Example_2D/CMakeLists.txt:5 (find_package)
100-- Configuring incomplete, errors occurred!

I believe this is due to the change from CMAKE_CURRENT_BINARY_DIR to CMAKE_BINARY_DIR in a046846a. Going back should fix it (let's see...).