To avoid the deprecation warning.

This fixes some build systems which look up the location of pg_config
via the location of the postgres binary itself, e.g. timescaledb,
instead of calling pg_config which is on PATH.

Since the -dev output is correctly placed before the default output of
postgresql in PATH, we can rely on that and call "pg_config" from the
default output's fake script. Only do that, when the one on PATH is
actually a different file, though, to prevent infinite loops.

Resolves NixOS#341408