install/ostree: Set zipl as the bootloader for s390x

`zipl` cannot dynamically read BLS config at boot, and requires the MBR
to be updated before. So we set `zipl` as the bootloader for s390x by
default. Otherwise, s390x systems cannot reboot into new deployments.

See #2151

Author: jbtrystram

crates/lib/src/install.rs

@@ -957,6 +957,22 @@ async fn initialize_ostree_root(state: &State, root_setup: &RootSetup) -> Result
             .run_capture_stderr()?;
     }
 
+    // For s390x, we set zipl as the bootloader
+    if cfg!(target_arch = "s390x") {
+        Command::new("ostree")
+            .args([
+                "config",
+                "--repo",
+                "ostree/repo",
+                "set",
+                "sysroot.bootloader",
+                "zipl",
+            ])
+            .cwd_dir(rootfs_dir.try_clone()?)
+            .run_capture_stderr()
+            .context("Setting bootloader config to zipl")?;
+    }
+
     let sysroot = {
         let path = format!(
             "/proc/{}/fd/{}",