Fixes <https://bugs.gnu.org/35582>.
Reported by sirgazil <sirgazil@zoho.com>.
Previously, leading zeros would be removed, leading to an "invalid"
UUID:
(uuid->string (uuid "00CA-050E" 'fat32))
⇒ "CA-50E"
(string->uuid "CA-50E" 'fat32)
⇒ #f
* gnu/system/uuid.scm (fat-uuid->string): Pad digits with zeros.
* tests/uuid.scm ("uuid, FAT32, leading zeros preserved"): New test.