gnu: ld-wrapper: Add '-rpath' flag only for libraries that are in the store.

This avoids adding bogus entries to the RUNPATH of installed binaries,
pointing to the build directory or similar.

* gnu/packages/ld-wrapper.scm (store-file-name?): New procedure.
  (rpath-arguments): Add "-rpath" flag on when FILE matches
  'store-file-name?', not when it matches 'pure-file-name?'.