Require a truthy value to enable TUIST_USE_SWIFTERPM

Setting TUIST_USE_SWIFTERPM no longer opts you into the SwifterPM path by mere presence. Only truthy values such as 1, true, TRUE, yes, or YES activate it, while values like 0 now correctly fall back to the standard Swift Package Manager resolution. This makes disabling SwifterPM through the environment variable behave as the flag name suggests.