build-unit-test-docker: Use separate build directory from source
build-unit-test-docker: Use separate build directory from source

googletest should use a separate build directory from the source
directory like all of the other cmake builds.

Change-Id: If69f2333ea056e905742157efc24823b5b597bc6
Signed-off-by: William A. Kennington III <30fff5198f4d10ca1117e5f3ec82f3b4b8900d81@google.com>

openbmc/openbmc-build-scripts,openbmc/openbmc-build-scripts