Forward custom environment variables to manifest evaluation

You can now declare additional environment variables that Tuist should forward to Project.swift, Workspace.swift, Package.swift, and project description helpers. Add a manifestEnvironment list inside Tuist.GenerationOptions in Tuist.swift, using exact names or prefix globs ending in *. Forwarded variables are included in the manifest cache hash, so changing their values invalidates the cache. This removes the need to mirror every custom variable behind a TUIST_ prefix when package manifests need environment values during both tuist install and tuist generate.