Removes unused variable from nix-build/nix-shell

This particular `shell` variable wasn't used, since a new one was
declared in the only side of the `if` branch that used a `shell`
variable.

It could realistically confuse developers thinking it could use `$SHELL`
under some situations.

src/nix-build/nix-build.cc

@@ -85,7 +85,6 @@ void mainWrapped(int argc, char * * argv)
     BuildMode buildMode = bmNormal;
     bool readStdin = false;
 
-    auto shell = getEnv("SHELL", "/bin/sh");
     std::string envCommand; // interactive shell
     Strings envExclude;