build: Fix zipball build after 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

.github/workflows/create-zipball.py

@@ -106,7 +106,12 @@ def main():
 		subprocess.check_call(['composer', 'install', '--no-dev', '--optimize-autoloader'])
 
 		# fill archive with data
-		with ZipFile(source_dir / filename, 'w', zipfile.ZIP_DEFLATED) as archive:
+		with ZipFile(
+			source_dir / filename,
+			'w',
+			zipfile.ZIP_DEFLATED,
+			strict_timestamps=False,
+		) as archive:
 			archive.prefix = Path('entries')
 
 			# Assets are copied by Webpack to www/dist/.