Looks like Jenkins is broken again, but this time for uboot, so manually download it instead:
https://arm01.puri.sm/job/u-boot_builds/job/uboot_librem5_mainline_build/lastSuccessfulBuild/artifact/output/uboot-librem5/u-boot-librem5.imx
Since the steps are manual now, download the Evergreen Byzantium LUKS image as well:
https://storage.puri.sm/librem5/images/byzantium/2024.04/librem5r4/luks/artifact/librem5r4.img.xz
Extract it to ~/librem5-flash-image, move u-boot-librem5.imx to that directory, then execute this command within the directory instead:
./scripts/librem5-flash-image --stable --skip-download --dir ./