Change buildimage-baseimage-pr cache mode to rcache (sonic-net#200)

Author: xumia

jenkins/generic/buildimage-baseimage-pr/Jenkinsfile

@@ -29,7 +29,7 @@ pipeline {
 
 git submodule foreach --recursive '[ -f .git ] && echo "gitdir: $(realpath --relative-to=. $(cut -d" " -f2 .git))" > .git'
 
-CACHE_OPTIONS="SONIC_DPKG_CACHE_METHOD=wcache SONIC_DPKG_CACHE_SOURCE=/nfs/dpkg_cache/generic"
+CACHE_OPTIONS="SONIC_DPKG_CACHE_METHOD=rcache SONIC_DPKG_CACHE_SOURCE=/nfs/dpkg_cache/generic"
 make configure PLATFORM=generic
 
 make SONIC_CONFIG_BUILD_JOBS=1 $CACHE_OPTIONS target/docker-ptf.gz target/sonic-generic.bin target/sonic-aboot-generic.swi