fix(cli): require truthy SwifterPM flag

Summary

• Require TUIST_USE_SWIFTERPM to use the shared truthy environment-variable check instead of enabling SwifterPM by mere presence.
• Preserve existing truthy values like 1, true, TRUE, yes, and YES through Environment.isVariableTruthy.
• Add a regression test proving TUIST_USE_SWIFTERPM=0 falls back to the standard swift package path.

Testing

• tuist install
• tuist generate tuist TuistSupport TuistSupportTests ProjectDescription --no-open
• xcodebuild test -workspace Tuist.xcworkspace -scheme Tuist-Workspace -only-testing TuistSupportTests/SwiftPackageManagerControllerTests/test_resolve_when_swifterpm_is_not_truthy_usesSwiftPackageManager CODE_SIGNING_ALLOWED=NO CODE_SIGNING_REQUIRED=NO CODE_SIGN_IDENTITY=""
• mise run cli:lint --fix