build: Prepare zipball build for Composer bump

Composer 2.6.4+ wants to have reproducible outputs so it preserves `mtime` when copying files:
composer/composer#11663
This becomes an issue with `vendor/composer/ClassLoader.php`,
which is copied from Nix store so it has `mtime` of 0 (1970-01-01),
because that is not supported by ZIP:

    ValueError: ZIP does not support timestamps before 1980

Let’s disable strict timestamps to clamp the out-of-range timestamps to ZIP epoch.
That argument was actually introduced because of reproducible builds:
https://bugs.python.org/issue34097

utils/create-zipball.py

@@ -165,6 +165,7 @@ def main() -> None:
             source_dir / filename,
             mode="w",
             compression=zipfile.ZIP_DEFLATED,
+            strict_timestamps=False,
         ) as archive:
             archive.prefix = Path("selfoss")