Use ftp.gnu.org for glibc-2.31 download.
Use ftp.gnu.org for glibc-2.31 download.

PiperOrigin-RevId: 337579675

GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker