Fix race condition that caused build-processing jobs to fail with a false “invalid SLF line” error

Build-processing jobs no longer collide on a shared download path. Previously, when two jobs ran concurrently for the same build (a duplicate-enqueue race), both wrote to the same temp file. One job’s download or cleanup could overwrite or delete the file the other was still reading, producing a spurious “doesn’t seem like a valid SLF line” error even though the stored archive was fine. Each job now uses a unique temp path, so concurrent runs of the same build can no longer corrupt each other’s local copies.