obs-mirror: keep nightly builds for two years

As we're moving the archive to a machine with sufficient storage,
let's extend the time we keep an archive of nightly builds from
one month to two years.

Change-Id: Ic8529ee5679f15c4e4940a5f095374605c2e5c79
Related: OS#6257
diff --git a/contrib/obs-mirror/rm-old-nightly-archives.sh b/contrib/obs-mirror/rm-old-nightly-archives.sh
index 485bf01..485788f 100755
--- a/contrib/obs-mirror/rm-old-nightly-archives.sh
+++ b/contrib/obs-mirror/rm-old-nightly-archives.sh
@@ -6,7 +6,7 @@
 DRY=0
 
 # Get removal date in seconds since epoch and display it
-DATE_RM_SEC=$(expr $(date +%s) - 3600 \* 24 \* 32)
+DATE_RM_SEC=$(expr $(date +%s) - 3600 \* 24 \* 365 \* 2)
 DATE_RM_STR=$(date -d "@$DATE_RM_SEC" +"%Y-%m-%d")
 echo "Removing nightly archives from $DATE_RM_STR and older (DRY=$DRY)"