postinstall: linux does not allow more than 64 bytes long hostnames

As per:

  $ getconf HOST_NAME_MAX
  64

truncate it to the maximum.

src/utils/postinstall.py

@@ -69,6 +69,10 @@ def set_windows_hostname(disk, partition, name):
 def set_linux_hostname(disk, partition, name):
     logging.info(f'Setting Linux hostname to {name}')
 
+    if len(name) > 64:
+        logging.warning(f'Linux does not permit hostnames that exceed 64 characters. Truncating {name}')
+        name = name[0:64]
+
     device = get_partition_device(disk, partition)
     mountpoint = device.replace('dev', 'mnt')