Add ability to override default path to postgresql install

[connorblack]

Hi - I'm currently unable to use pgx after switching over to a different postgresql image (bitnami) that installs postgres in /opt/bitnami/postgresql instead of /usr/lib/postgresql. It would be great to have a flag you can pass to override the path.

https://github.com/pgxman/pgxman/blob/5f197fcb9deb17d416c3c7def7a42c36a15b4b26/internal/pg/version.go#L26C23-L26C42:

func VersionExists(ctx context.Context, ver pgxman.PGVersion) bool {
	path := fmt.Sprintf("/usr/lib/postgresql/%s/bin/pg_config", ver)
	pgVer, err := pgConfigVersion(ctx, path)
	if err != nil {
		return false
	}

	return pgVer == ver
}