[Fix] Zip spec compliance for exported backup file

[brandonpayton]

Motivation for the change, related issues

Files in a zip file should only have relative urls as per spec. Right now, when exporting the backup for the site, we create a zip file with a leading slash in the files inside it.

This got surfaced in Studio app when we switched to a diff zip library that strictly adheres to zip spec and PG exported zip files fail to process.

Related to: https://github.com/Automattic/dotcom-forge/issues/10369

Note: There are other places where ZipArchive is used but this PR only modifies the location that impacts the site export.
Probably other places can use the same change, and I can follow that up in a separate PR, if you like.

Testing Instructions (or ideally a Blueprint)

Export zip of a site and examine the file paths inside it using the following script:

<?php
$zip = new ZipArchive();
$zip->open('/Users/ashfame/Downloads/wordpress-playground-16.zip');
echo "=== ZIP Contents ===".PHP_EOL;
for($i = 0; $i < $zip->numFiles; $i++) {
	echo $zip->getNameIndex($i) . PHP_EOL;
}
echo "=== End ZIP Contents ===".PHP_EOL;
$zip->close();

Prior to this PR, you would see file paths like:

/wp-content/themes/twentytwentyfive/patterns/more-posts.php

and with this PR you will see file paths without the leading slash:

wp-content/themes/twentytwentyfive/patterns/more-posts.php