Only set DOCKER_CROSSPLATFORMS from env if it's set

This allows for the default to come from the image/`Dockerfile` rather
than being unconditionally overwritten by the environment.

Signed-off-by: Andrew "Tianon" Page <admwiggin@gmail.com>

Makefile

@@ -17,13 +17,13 @@ export DOCKER_GITCOMMIT
 # to allow things like `make KEEPBUNDLE=1 binary` easily
 # `project/PACKAGERS.md` have some limited documentation of some of these
 DOCKER_ENVS := \
+	$(if $(DOCKER_CROSSPLATFORMS), -e DOCKER_CROSSPLATFORMS) \
 	-e BUILD_APT_MIRROR \
 	-e BUILDFLAGS \
 	-e KEEPBUNDLE \
 	-e DOCKER_BUILD_ARGS \
 	-e DOCKER_BUILD_GOGC \
 	-e DOCKER_BUILD_PKGS \
-	-e DOCKER_CROSSPLATFORMS \
 	-e DOCKER_DEBUG \
 	-e DOCKER_EXPERIMENTAL \
 	-e DOCKER_GITCOMMIT \