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.