use the correct path to include Compiler.jl in release builds (#56601)
https://github.com/JuliaLang/julia/pull/56409 broke PackageCompiler (or
other use cases where you want to compile a new core compiler from a
release build) since it hardcoded the relative path `../usr/` from Base
to the `shared` directory but this is not true in releases where it is
at `..`.