install: improve error message when systemd-boot is selected without composefs backend

### Summary

When `--bootloader=systemd` is specified without `--composefs-backend`, the error message is:

```
error: Installing to disk: bootupd is required for ostree-based installs
```

This doesn't tell the user what to do. The fix is to use `--composefs-backend`, but the error message doesn't mention composefs at all. It reads like bootupd is a hard requirement, when actually systemd-boot works fine with the composefs backend.

### Reproduction

```bash
podman run --rm --privileged --pid=host \
-v /dev:/dev -v /var/lib/containers:/var/lib/containers \
quay.io/fedora/fedora-bootc:42 \
bootc install to-disk --wipe --bootloader systemd --filesystem xfs /dev/sdX
```

### Suggested improvement

Something like:

```
error: systemd-boot is not supported with the ostree backend. Use --composefs-backend to enable systemd-boot support.
```

Or if the user explicitly asked for systemd-boot, auto-enable composefs (with a note).

### Context

The current error is at `install.rs:1818`. The bail message is technically accurate but not actionable for users who don't know about the composefs/ostree backend distinction.