Avoid constructing toolchain in std.wrap prematurely

[deciduously]

The current implementation proactively selects a build toolchain before it knows whether the specific call in question will require one. This causes issues in bootstrap situations where one must provide a bootstrap toolchain to std.wrap() to avoid this, even if it will not be used. This should be refactored not to attempt to produce a toolchain for wrappers that require no compilation of additional components to construct.