abuild: use stat instead of df to figure filesystem type

Fixes the error:
  df: .: can't find mount point

When running abuild inside a chroot when the root file system
mountpoint information is not necessarily directly available.

abuild.in

@@ -997,7 +997,7 @@ prepare_metafiles() {
 	local builddate="$SOURCE_DATE_EPOCH"
 
 	# Fix package size on several filesystems
-	case "$(df -PT . | awk 'END {print $2}')" in
+	case "$(stat -f -c "%T" .)" in
 	btrfs|ecryptfs|zfs)
 		sync;;
 	esac