Filesystem: Use a more unique filename in `wp_tempnam()` and `get_filesystem_method()`.

Using a filename which was generated from `time()` could cause two processes to try to use the same filename, causing unexpected behaviour.

Props jrchamp, bikecrazyy.
Fixes #42265.

Built from https://develop.svn.wordpress.org/trunk@42224


git-svn-id: https://core.svn.wordpress.org/trunk@42053 1a063a9b-81f0-0310-95a4-ce76da25c4cd