For scripts, use DYLD_FRAMEWORK/LIBRARY_PATH to find frameworks / runtimes.
Add DYLD_FRAMEWORK_PATH=/System/Library/Frameworks to the environment when
constructing swift-frontend invocations for the interpreter so that dlopen can
find autolinked frameworks. This is intended to fix
https://github.com/apple/swift/issues/68785.
As a follow-up: don't expand runtime paths to find the path to the runtime in
libImmediate, but instead hard-code "/usr/lib/swift/libswiftCore.dylib" and
allow any overrides to be found via DYLD_LIBRARY_PATH (this ensures that dyld
treats whichever libswiftCore.dylib is found as override for the one in the
shared cache, which doesn't happen if dlopen is passed a non-standard path).