android: allow override of FRESH_CLONE target directory

my /tmp is not large enough, this allows override of build location using

export FRESH_CLONE=/some/other/location

contrib/android/build.sh

@@ -51,7 +51,7 @@ docker build \
 # maybe do fresh clone
 if [ ! -z "$ELECBUILD_COMMIT" ] ; then
     info "ELECBUILD_COMMIT=$ELECBUILD_COMMIT. doing fresh clone and git checkout."
-    FRESH_CLONE="/tmp/electrum_build/android/fresh_clone/electrum"
+    FRESH_CLONE=${FRESH_CLONE:-"/tmp/electrum_build/android/fresh_clone/electrum"}
     rm -rf "$FRESH_CLONE" 2>/dev/null || ( info "we need sudo to rm prev FRESH_CLONE." && sudo rm -rf "$FRESH_CLONE" )
     umask 0022
     git clone "$PROJECT_ROOT" "$FRESH_CLONE"