ci/bare-metal: Include a timestamp in our serial reads.

gitlab CI doesn't include timestamps in its logs by default, but it's
really useful for finding delays in our CI so stuff one in on the lines
coming in from serial and being output to the gitlab log.  The artifacts
file is still the raw serial output.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6529>

.gitlab-ci/bare-metal/serial_buffer.py

@@ -22,6 +22,7 @@
 # IN THE SOFTWARE.
 
 import argparse
+from datetime import datetime,timezone
 import queue
 import serial
 import threading
@@ -76,7 +77,10 @@ class SerialBuffer:
                 line.append(b)
                 if b == b'\n'[0]:
                     line = line.decode(errors="replace")
-                    print(self.prefix + line, flush=True, end='')
+
+                    time = datetime.now().strftime('%y-%m-%d %H:%M:%S')
+                    print("{time} {prefix}{line}".format(time=time, prefix=self.prefix, line=line), flush=True, end='')
+
                     self.line_queue.put(line)
                     line = bytearray()