keep opam's bin/ at the same place in the PATH list during a switch

The proposed patch will keep the `bin/` directory corresponding to OPAM compiler's version at the same place in the `PATH` list when running `opam config env` after a switch. If no such directory was present in `PATH`, it is appended as the beginning of the list as is the case now. In the current setup, OPAM's `bin/` dir will be put in front of `PATH` regardless of its actual position, meaning that an `opam switch` may result in OPAM-managed executables inadvertently shadowing versions installed outside of OPAM.