set-source-date-epoch-to-latest: fix warning

If there was no older file than $NIX_BUILD_TOP this would result in a
warning, e.g. with nix-info.

```
/nix/store/15kgcm8hnd99p7plqzx7p4lcr2jni4df-set-source-date-epoch-to-latest.sh: line 13: [: : integer expression expected
```

pkgs/build-support/setup-hooks/set-source-date-epoch-to-latest.sh

@@ -10,7 +10,7 @@ updateSourceDateEpoch() {
     local newestFile="${res[1]}"
 
     # Update $SOURCE_DATE_EPOCH if the most recent file we found is newer.
-    if [ "$time" -gt "$SOURCE_DATE_EPOCH" ]; then
+    if [ "${time:-0}" -gt "$SOURCE_DATE_EPOCH" ]; then
         echo "setting SOURCE_DATE_EPOCH to timestamp $time of file $newestFile"
         export SOURCE_DATE_EPOCH="$time"