clangOmpss2: use relative path for BINDIR

Absolute paths are not supported and cause a silent error when
installing the clang++ and other symlinks. See:

d93a11c138
58580e922a

bsc/llvm-ompss2/clang.nix

@@ -70,6 +70,7 @@ stdenv.mkDerivation rec {
       "-DLLVM_ENABLE_PROJECTS=clang;openmp;compiler-rt"
       "-DLLVM_ENABLE_ASSERTIONS=${enableAssertions}"
       "-DLLVM_INSTALL_TOOLCHAIN_ONLY=ON"
+      "-DCMAKE_INSTALL_BINDIR=bin"
     )
   '';