Add BB_ENV_EXTRAWHITE

ci/conf/ci.conf

 # === SETTINGS FOR PROJECT OR SOURCECODE ===
 
 # Where to save build caches
+export BB_ENV_EXTRAWHITE="DL_DIR SSTATE_DIR"
 export SSTATE_DIR="${HOME}/yocto-common/rocko/sstate-cache"
 export DL_DIR="${HOME}/yocto-common/rocko/downloads"