CMake: avoid incorrect extra "" around the GOOGLETEST_PATH.
CMake: avoid incorrect extra "" around the GOOGLETEST_PATH.

It was looking in `"<...>/googlebenchmark/build/third_party/googletest"`,
with these extra `"` quotes, and wasn't finding anything..

google/benchmark,google/benchmark,mockingbirdnest/benchmark,mockingbirdnest/benchmark,mockingbirdnest/benchmark,mockingbirdnest/benchmark,google/benchmark