[CMake] Always use lib as library directory in builtin CFITSIO prefix

Like this, we make sure the static library is where we expect it to be:
`CFITSIO-prefix/lib/libcfitsio.a`, and not for example in the `lib64`
directory.

Inspired by this forum post:
https://root-forum.cern.ch/t/6-32-02-will-not-build-from-source-with-option-dbuiltin-cfitsio/60270

builtins/cfitsio/CMakeLists.txt

@@ -30,6 +30,7 @@ ExternalProject_Add(
              -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
              -DCMAKE_C_FLAGS=${CFITSIO_C_FLAGS}
              -DCMAKE_INSTALL_PREFIX=<INSTALL_DIR>
+             -DCMAKE_INSTALL_LIBDIR=<INSTALL_DIR>/lib
              -DBUILD_SHARED_LIBS=OFF
              -DUSE_CURL=OFF
              -DZLIB_FOUND=TRUE