Swift: Allow build and installed RPATHs to differ

Define the RPATH separator to `:` so that CMake can reserve room to edit
the ELF RPATH at install time.  This enables the use of `BUILD_RPATH` and
`INSTALL_RPATH` with differing values.
8 jobs for separator
Status Name Job ID Coverage
  External
passed ghostflow-check-master

passed ghostflow-check-release

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager