fix: append pip errors to log file rather than overwrite

Resolves pypa#1132 

An alternative would be to create new log files for each pip invocation, but this would require deeper changes. Appending seems like a decent solution as a first fix.

src/pipx/util.py

@@ -324,7 +324,7 @@ def subprocess_post_check_handle_pip_error(
         if pipx.constants.pipx_log_file is None:
             raise PipxError("Pipx internal error: No log_file present.")
         pip_error_file = pipx.constants.pipx_log_file.parent / (pipx.constants.pipx_log_file.stem + "_pip_errors.log")
-        with pip_error_file.open("w", encoding="utf-8") as pip_error_fh:
+        with pip_error_file.open("a", encoding="utf-8") as pip_error_fh:
             print("PIP STDOUT", file=pip_error_fh)
             print("----------", file=pip_error_fh)
             if completed_process.stdout is not None: