project: use --netrc-optional instead of --netrc

Some users are reporting a "curl: (26) .netrc error: no such file"
message on sync caused by an change to curl behavior.
See https://github.com/curl/curl/issues/16163.

Use --netrc-optional which was introduced in curl version 7.9.8
released in 2002.

Bug: 409354839
Change-Id: I8365c6e806968a4ee765a7e023b4bced30489c20
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/467026
Reviewed-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>

project.py

@@ -2875,7 +2875,14 @@ class Project:
 
         # We do not use curl's --retry option since it generally doesn't
         # actually retry anything; code 18 for example, it will not retry on.
-        cmd = ["curl", "--fail", "--output", tmpPath, "--netrc", "--location"]
+        cmd = [
+            "curl",
+            "--fail",
+            "--output",
+            tmpPath,
+            "--netrc-optional",
+            "--location",
+        ]
         if quiet:
             cmd += ["--silent", "--show-error"]
         if os.path.exists(tmpPath):