Disable building Proton in Gitian

This is the patch used to build 1.0.9 and 1.0.10.

Part of #2404.

contrib/gitian-descriptors/gitian-linux.yml

@@ -85,7 +85,7 @@ script: |
   BASEPREFIX=`pwd`/depends
   # Build dependencies for each host
   for i in $HOSTS; do
-    make ${MAKEOPTS} -C ${BASEPREFIX} HOST="${i}"
+    NO_PROTON="x" make ${MAKEOPTS} -C ${BASEPREFIX} HOST="${i}"
   done
 
   # Faketime for binaries