Set backup MANPATH in case man path isn’t set correctly.

Previously, this would fail at startup for non-NixOS installs:

nix-env --help

The fix for this is to just use "nixManDir" as the value for MANPATH
when spawning "man".

To test this, I’m using the following:

$ nix-build release.nix -A build
$ MANPATH= ./result/bin/nix-env --help

Fixes #1627

src/libmain/shared.cc

@@ -262,6 +262,7 @@ void printVersion(const string & programName)
 void showManPage(const string & name)
 {
     restoreSignals();
+    setenv("MANPATH", settings.nixManDir.c_str(), 1);
     execlp("man", "man", name.c_str(), NULL);
     throw SysError(format("command 'man %1%' failed") % name.c_str());
 }