Function bootupd_supports_filesystem 

fn bootupd_supports_filesystem(
    rootfs: &Utf8Path,
    deployment_path: Option<&str>,
) -> Result<bool>

Check whether the target bootupd supports --filesystem.

Runs bootupctl backend install --help and looks for --filesystem in the output. When deployment_path is set the command runs inside a bwrap container so we probe the binary from the target image.