[build] Adjust install name for Differentiation and Concurrency (#34216)

This effectively reverts #31183 -- we need to match the install name convention of the other stdlib libraries.

From the review feedback:

> The right way to load the stdlib & runtime libraries from a custom toolchain is to set `DYLD_LIBRARY_PATH` when executing the generated binary. This is how we run tests against the just-built libraries and this is how downloadable toolchain snapshots are currently configured in Xcode -- see #33178

Author: edymtt

stdlib/public/Concurrency/CMakeLists.txt

@@ -29,5 +29,4 @@ add_swift_target_library(swift_Concurrency ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} I
     ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
     -parse-stdlib
   LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}"
-  DARWIN_INSTALL_NAME_DIR "@rpath"
   INSTALL_IN_COMPONENT stdlib)

stdlib/public/Differentiation/CMakeLists.txt

@@ -38,5 +38,4 @@ add_swift_target_library(swift_Differentiation ${SWIFT_STDLIB_LIBRARY_BUILD_TYPE
     ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
     -parse-stdlib
   LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}"
-  DARWIN_INSTALL_NAME_DIR "@rpath"
   INSTALL_IN_COMPONENT stdlib)