Move savedArgv into libmain

`savedArgv` is not accessible by plugins when defined in main binary.
Moving it into one of the nix lib fix the problem.

src/libmain/shared.cc

@@ -33,6 +33,7 @@
 
 namespace nix {
 
+char * * savedArgv;
 
 static bool gcWarning = true;
 

src/nix/main.cc

@@ -53,7 +53,6 @@ static bool haveInternet()
 }
 
 std::string programPath;
-char * * savedArgv;
 
 struct HelpRequested { };