wireshark: run binary from nix-shell

when in a shell, export variable WIRESHARK_RUN_FROM_BUILD_DIRECTORY to be able to run the newly built wireshark
else one get:
./build/run/wireshark: error while loading shared libraries: libwscodecs.so.0: cannot open shared object file: No such file or directory

see https://github.com/NixOS/nixpkgs/issues/29638