autoPatchelf: dont add workdir to rpath

[milahu]

setting interpreter of native/linux/x64/tor.tar.xz/tor
searching for dependencies of native/linux/x64/tor.tar.xz/tor
    libz.so.1 -> found: /nix/store/9nk7bsdlsmmnj96bivbvgqy491p65jdq-libz-1.2.8.2015.12.26-unstable-2018-03-31/lib
    libevent-2.1.so.7 -> found: /build/source/native/linux/x64/tor.tar.xz
    libssl.so.3 -> found: /build/source/native/linux/x64/tor.tar.xz
    libcrypto.so.3 -> found: /build/source/native/linux/x64/tor.tar.xz
setting RPATH to: /nix/store/9nk7bsdlsmmnj96bivbvgqy491p65jdq-libz-1.2.8.2015.12.26-unstable-2018-03-31/lib:/build/source/native/linux/x64/tor.tar.xz

the executable tor and the dependencies libevent-2.1.so.7 ... are in the same directory

the path /build/source/native/linux/x64/tor.tar.xz/ is only valid on build time, but not on run time

adding workdir to rpath is ok if the path is also valid on run time
in effect, if workdir is in /nix/store