sd-image: optimise sfdisk call for image building

When building an image, rather than operating on a real partition,
there's no need to have sfdisk tell the kernel to reread the partition
table.  This also avoids a call to sync(2), which could be very
expensive depending on what other IO is going on on the system at the time.

nixos/modules/installer/sd-card/sd-image.nix

@@ -209,7 +209,7 @@ in
         # type=b is 'W95 FAT32', type=83 is 'Linux'.
         # The "bootable" partition is where u-boot will look file for the bootloader
         # information (dtbs, extlinux.conf file).
-        sfdisk $img <<EOF
+        sfdisk --no-reread --no-tell-kernel $img <<EOF
             label: dos
             label-id: ${config.sdImage.firmwarePartitionID}