helios-build -o build failed with a CPIO error

@grwilson was running a helios-build image -o build -N ??? (I don't remember the -N value) and it failed with:

Dec 05 19:47:32.028 INFO creating Oxide boot image...
Dec 05 19:47:32.028 INFO exec: ["/home/gwilson/helios/projects/bootserver/target/release/mkimage", "-i", "/rpool/images/gwilson/output/sled-zfs.raw", "-N", "cosmo-b-120525_0115", "-o", "build/zfs.img", "-O", "/home/gwilson/helios/tmp/image/boot_image_csum", "-s", "4096"], pwd: None
Dec 05 19:47:33.004 INFO O| ok, image written!
Dec 05 19:47:33.004 INFO O| ok, boot_image_csum file written!
Dec 05 19:47:33.008 INFO creating boot archive (CPIO)...
Dec 05 19:47:33.008 INFO exec: ["bash", "/home/gwilson/helios/image/mkcpio.sh", "/rpool/images/gwilson/work/sled/ramdisk", "build/cpio", "/home/gwilson/helios/tmp/image"], pwd: None
Dec 05 19:47:33.014 INFO E| + root=/rpool/images/gwilson/work/sled/ramdisk
Dec 05 19:47:33.014 INFO E| + outfile=build/cpio
Dec 05 19:47:33.014 INFO E| + tmpdir=/home/gwilson/helios/tmp/image
Dec 05 19:47:33.016 INFO E| + cd /rpool/images/gwilson/work/sled/ramdisk
Dec 05 19:47:33.017 INFO E| + cpio -qo -H odc -O build/cpio
Dec 05 19:47:33.019 INFO E| cpio: Cannot open "build/cpio" for output
Dec 05 19:47:33.019 INFO E| USAGE:
Dec 05 19:47:33.019 INFO E| cpio -i[bcdfkmqrstuv@BSV6] [-C size] [-E file] [-H hdr] [-I file [-M msg]] [-R id] [patterns]
Dec 05 19:47:33.019 INFO E| cpio -o[acv@ABLV] [-C size] [-H hdr] [-O file [-M msg]]
Dec 05 19:47:33.019 INFO E| cpio -p[adlmuv@LV] [-R id] directory
Error: exec ["bash", "/home/gwilson/helios/image/mkcpio.sh", "/rpool/images/gwilson/work/sled/ramdisk", "build/cpio", "/home/gwilson/helios/tmp/image"]: failed ExitStatus(unix_wait_status(256))

Start with image/mkcpio.sh and the helios-build image path that invokes it; inspect how the output path is handled after changing into the ramdisk. Reproduce the command with the paths shown in the report and confirm that a CPIO archive is produced without the reported error.