Provide rootfs url for build nodeid

Build artifacts (kernel, dtb, modules) are provided by the build node id. However this does not include the rootfs url. It would be helpful if the rootfs url were also added to the artifacts as this would help in reproducing issues.

```
> kci-dev maestro-results --nodeid 67b815833233578a92935ea7
[
{
"artifacts": {
"_config": "http://mon.kernelci.org:3000/kbuild-gcc-12-x86-67b815833233578a92935ea7/.config",
"build_kimage_log": "http://mon.kernelci.org:3000/kbuild-gcc-12-x86-67b815833233578a92935ea7/build_kimage.log.gz",
"build_kimage_stderr_log": "http://mon.kernelci.org:3000/kbuild-gcc-12-x86-67b815833233578a92935ea7/build_kimage_stderr.log.gz",
"build_kselftest_log": "http://mon.kernelci.org:3000/kbuild-gcc-12-x86-67b815833233578a92935ea7/build_kselftest.log.gz",
"build_kselftest_stderr_log": "http://mon.kernelci.org:3000/kbuild-gcc-12-x86-67b815833233578a92935ea7/build_kselftest_stderr.log.gz",
"build_log": "http://mon.kernelci.org:3000/kbuild-gcc-12-x86-67b815833233578a92935ea7/build.log.gz",
"build_modules_log": "http://mon.kernelci.org:3000/kbuild-gcc-12-x86-67b815833233578a92935ea7/build_modules.log.gz",
"build_modules_stderr_log": "http://mon.kernelci.org:3000/kbuild-gcc-12-x86-67b815833233578a92935ea7/build_modules_stderr.log.gz",
"build_sh": "http://mon.kernelci.org:3000/kbuild-gcc-12-x86-67b815833233578a92935ea7/build.sh",
"fragments/0_config": "http://mon.kernelci.org:3000/kbuild-gcc-12-x86-67b815833233578a92935ea7/fragments/0.config",
"fragments/1_config": "http://mon.kernelci.org:3000/kbuild-gcc-12-x86-67b815833233578a92935ea7/fragments/1.config",
"fragments/2_config": "http://mon.kernelci.org:3000/kbuild-gcc-12-x86-67b815833233578a92935ea7/fragments/2.config",
"fragments/3_config": "http://mon.kernelci.org:3000/kbuild-gcc-12-x86-67b815833233578a92935ea7/fragments/3.config",
"kernel": "http://mon.kernelci.org:3000/kbuild-gcc-12-x86-67b815833233578a92935ea7/bzImage",
"kselftest_tar_gz": "http://mon.kernelci.org:3000/kbuild-gcc-12-x86-67b815833233578a92935ea7/kselftest.tar.gz",
"metadata": "http://mon.kernelci.org:3000/kbuild-gcc-12-x86-67b815833233578a92935ea7/metadata.json",
"modules": "http://mon.kernelci.org:3000/kbuild-gcc-12-x86-67b815833233578a92935ea7/modules.tar.xz",
"vmlinux": "http://mon.kernelci.org:3000/kbuild-gcc-12-x86-67b815833233578a92935ea7/vmlinux.xz"
},
```

Cc: @nuclearcat @padovan