[bootstrap] Add the runtime library paths to [DY]LD_LIBRARY_PATHS…

[mikeash]

…so we use the just-built libswiftCore instead of the OS's copy.

[mikeash]

@neonichu I hit a problem where SwiftPM failed when building with build-script --swift-inspect --infer due to loading the OS's libswiftCore.dylib instead of the one we just built. This fixes it by adding the appropriate paths to [DY]LD_LIBRARY_PATHS. Is this the right fix for this issue?

[neonichu]

This makes sense to me.

It looks like we're already using the runtime library paths as a source for rpaths here: https://github.com/apple/swift-package-manager/blob/839a5b668a3ffebce5c673214a7a96a01520af3a/Utilities/bootstrap#L659, but obviously that won't change the link time behaviour.

I will double check locally how the paths look like and whether it would make sense to move the more complex logic that is used for computing rpaths to also compute linker search paths.

[neonichu]

@swift-ci please smoke test