After installing CDT, WasmToolchain still uses toolchain in build directory

[nathanielhourt]

After installing the CDT, i.e. to /opt/eosio-cdt, the CMake modules in the installed path still contain references to the toolchain in the build root. For example, look in /opt/eosio-cdt/lib/cmake/eosio.cdt/EosioWasmToolchain.cmake, and you'll see it sets the compilers etc to the ones (probably no longer) in the build directory rather than the proper ones in /opt/eosio-cdt/eosio.cdt/bin. Contract builds fail unless the build directory is preserved.