Microfabricated alkali-vapor cells with tunable <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>He</mml:mi> </mml:math> - <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>Ne</mml:mi> </mml:math> buffer-gas mixture using reservoirs with laser-actuated break seals