Require a truthy value for the TUIST_USE_SWIFTERPM flag

When opting into SwifterPM via the TUIST_USE_SWIFTERPM environment variable, the CLI now treats the variable as truthy only when its value is actually truthy (for example 1, true, TRUE, yes, or YES), instead of treating any presence of the variable as a positive signal. Setting TUIST_USE_SWIFTERPM=0 or other falsy values now correctly falls back to the standard Swift Package Manager path, matching how other CLI switches behave.