Adjust wal fast source logic to explicitly connsider data directory

Simplified logic of wal_fast can be found here [1] which is as follows:
```
WAL_FAST=./wal_fast
```
Hence, aligning with the same logic to keep things simple. This way, it will not conflict
with separate WAL directory too.

[1] https://github.com/anasanjaria/spilo/blob/master/postgres-appliance/scripts/basebackup.sh

Author: anasanjaria

Diff for: postgres-appliance/scripts/restore_command.sh

@@ -27,7 +27,7 @@ readonly wal_destination=$2
 
 wal_dir=$(dirname "$wal_destination")
 readonly wal_dir
-wal_fast_source=$(dirname "$(dirname "$(realpath "$wal_dir")")")/wal_fast/$wal_filename
+wal_fast_source=$(dirname "$PGDATA/wal_fast/$wal_filename")
 readonly wal_fast_source
 
 [[ -f $wal_fast_source ]] && exec mv "${wal_fast_source}" "${wal_destination}"