Fix flash target in contrib/Makefile

Setting the working directory to a directory that exist.

Previously the working directory was set to /build/hw/application_fpga.
But that path does not exist since hw/application_fpga was mounted to
/build. Thus resulting in an error:
'Error: workdir "/build/hw/application_fpga" does not exist on
container'

contrib/Makefile

@@ -53,7 +53,7 @@ flash:
 	podman run --rm \
 	--device /dev/bus/usb/$(lsusb | grep -m 1 1209:8886 | awk '{ printf "%s/%s", $2, substr($4,1,3) }') \
 	--mount type=bind,source="`pwd`/../hw/application_fpga",target=/build \
-	-w /build/hw/application_fpga \
+	-w /build \
 	-it $(IMAGE) tillitis-iceprog /build/application_fpga.bin