SwifterPM flag now requires a truthy value

The opt-in environment variable TUIST_USE_SWIFTERPM now requires a truthy value (for example, 1, true, yes, or YES) to enable SwifterPM. Previously, setting the variable to any value, including 0 or false, would enable SwifterPM. This makes the flag behave consistently with other Tuist environment toggles and avoids unexpected behavior when the variable is set to a non-truthy value.