set CARGO_HOME to $SRCDEST/cargo

CARGO_HOME tells cargo where to store installed dependencies, save it to
a directory in $SRCDEST so we don't need to download all dependencies
again when compiling a rust package.

abuild.in

@@ -2643,6 +2643,7 @@ aportsgit=${APORTSDIR:-${startdir}}
 repo_template=$aportsgit/$repo/.rootbld-repositories
 
 SRCDEST=${SRCDEST:-$startdir}
+CARGO_HOME=$SRCDEST/cargo
 
 BUILD_ROOT=
 
@@ -2661,7 +2662,7 @@ if [ -z "$REPODEST" ]; then
 fi
 
 # for recursive action
-export REPODEST SRCDEST
+export REPODEST SRCDEST CARGO_HOME
 
 # add dbg subpackage if its enabled globally
 if [ -n "$DEFAULT_DBG" ] && ! subpackage_types_has "dbg" && ! options_has "!dbg" && [ "$arch" != "noarch" ]; then