These changes are necessary to make download-rustc more stable and less risky:

• Based on this idea from @RalfJung, instead of checking specific directories to see if they have changed, check for any changes outside of certain allowed directories. This is more maintainable over time because the current approach requires constantly updating hard-coded denied paths.
• When not running in CI, if the "library" tree has changed but the "compiler" tree hasn't, avoid recompiling the "compiler" tree. This approach is similar to how download-rustc=true handles library changes. The goal is to make if-unchanged useful for library developers as well.
• Once above is done, make download-rustc=if-unchanged default option for the "library" profile.
• download-rustc has no effect when running run-make tests #131947