Unlike /etc/hostname, this should always be present (#92)

Should fix: #84

I had thought `/etc/hostname` was universal - it is not, notably on some
`GCP` instances.

`/proc/sys/kernel/hostname` should be, however.

Author: bleggett

src/preinstall_cmd.rs

@@ -125,7 +125,7 @@ pub async fn do_preinstall(
     let mut all_groups_result = Passed;
 
     let hostname = host_executor
-        .spawn_in_host_ns(async { std::fs::read_to_string("/etc/hostname").unwrap() })
+        .spawn_in_host_ns(async { std::fs::read_to_string("/proc/sys/kernel/hostname").unwrap() })
         .await?;
 
     let base_dir = if let Some(dir) = report_dir {