Update README to include Poetry system Git client configuration workaround

[TinyMarsh]

This change addresses a HangupException error that occurs when Poetry's default Git client fails to work with the SSH configuration provided by this action. The solution ensures compatibility by switching to the system-installed Git client.

Relevant chunk of the stack trace below. This only seemed to be an issue on Windows runners:

  - Installing <private_repo> (0.1.0 68fd337)

  HangupException

  The remote server unexpectedly closed the connection.

  at C:\Program Files (x86)\pipx\venvs\poetry\lib\site-packages\dulwich\protocol.py:255 in read_pkt_line
      251| 
      252|         try:
      253|             sizestr = read(4)
      254|             if not sizestr:
    > 255|                 raise HangupException
      256|             size = int(sizestr, 16)
      257|             if size == 0 or size == 1:  # flush-pkt or delim-pkt
      258|                 if self.report_activity:
      259|                     self.report_activity(4, "read")

Cannot install <private_repo>.

Error: Process completed with exit code 1.