gitian: omit ccache from gitian builds

It's just wasted overhead since the build VMs are always deleted
and recreated fresh for each run.

contrib/gitian/gitian-linux.yml

@@ -36,7 +36,6 @@ packages:
 - "ca-certificates"
 - "python"
 - "cmake"
-- "ccache"
 - "protobuf-compiler"
 - "libdbus-1-dev"
 - "libharfbuzz-dev"
@@ -63,6 +62,7 @@ script: |
   if test -n "$GBUILD_CACHE_ENABLED"; then
     export SOURCES_PATH=${GBUILD_COMMON_CACHE}
     export BASE_CACHE=${GBUILD_PACKAGE_CACHE}
+    export GITIAN=1
     mkdir -p ${BASE_CACHE} ${SOURCES_PATH}
   fi