[build] Fix debian mirror snapshot timestamp not work issue. (#21758)

Why I did it
Recently microsoft owned debian mirror has an issue. It is using the latest timestamp '20250216'. And this snapshot is a bad snapshot. Bydesign we should use timestamp '20250130', which passed PR validation.

Get:18 http://packages.trafficmanager.net/snapshot/debian-security/20250216T001420Z bullseye-security/non-free amd64 Packages [1164 B]
Get:19 http://packages.trafficmanager.net/snapshot/debian-security/20250216T001420Z bullseye-security/main amd64 Packages [346 kB]
Fetched 17.9 MB in 5s (3789 kB/s)
Reading package lists...
E: Failed to fetch http://packages.trafficmanager.net/snapshot/debian-security/20250216T001420Z/dists/bullseye-security/main/source/Sources.xz  File has unexpected size (234932 != 234820). Mirror sync in progress? [IP: 13.107.246.71 80]
   Hashes of expected file:
    - Filesize:234820 [weak]
    - SHA256:99cbb6795504058c2b3b2355ad128d8ca7879c136ba8fc00f2813ea4d5e7d147
   Release file created at: Sat, 15 Feb 2025 17:04:32 +0000
E: Some index files failed to download. They have been ignored, or old ones used instead.
By design, we should use timestamp in https://github.com/sonic-net/sonic-buildimage/blob/master/files/build/versions/default/versions-mirror

How I did it
Fix debian mirror snapshot timestamp generate script.

Author: liushilongbuaa

scripts/build_mirror_config.sh

@@ -11,7 +11,7 @@ export APT_RETRIES_COUNT
 
 DEFAULT_MIRROR_URL_PREFIX=http://packages.trafficmanager.net
 MIRROR_VERSION_FILE=
-[[ "$SONIC_VERSION_CONTROL_COMPONENTS" == *deb* || $SONIC_VERSION_CONTROL_COMPONENTS == *all* ]] && MIRROR_VERSION_FILE=files/build/versions/default/versions-mirror
+[[ "$MIRROR_SNAPSHOT" == "y" ]] && MIRROR_VERSION_FILE=files/build/versions/default/versions-mirror
 [ -f target/versions/default/versions-mirror ] && MIRROR_VERSION_FILE=target/versions/default/versions-mirror
 
 # The default mirror urls