build-unit-test-docker: Add stdplus to the cache
build-unit-test-docker: Add stdplus to the cache

This will soon be needed by sdeventplus and gpioplus.

Change-Id: Ia67b6a95ed33f61003721e5aa927b96fe88bc8e1
Signed-off-by: William A. Kennington III <30fff5198f4d10ca1117e5f3ec82f3b4b8900d81@google.com>

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