Add explicit paths to commands in proxysql_galera_checker.sh

[pondix]

When using the Docker images, a failure message is displayed as shown below.

/usr/share/proxysql/tools/proxysql_galera_checker.sh: line 276: [: -eq: unary operator expected
/usr/share/proxysql/tools/proxysql_galera_checker.sh: line 308: [: too many arguments

A workaround for the meantime would require directly modifying the defined PATH variable used (i.e. runtime environment vars).