caching is working

* add almost all the input parameter state to the hash
   - missing items are the detected MSVC installation on Windows
     and detected libc installation on POSIX
   - also missing are C files and .h files that libclang finds
 * artifacts are created in global cache directory instead of
   zig-cache.
   - exception: builtin.zig is still in zig-cache
 * zig run uses the new cache correctly
 * zig run uses execv on posix systems

src/error.cpp

@@ -29,6 +29,7 @@ const char *err_str(int err) {
         case ErrorCCompileErrors: return "C compile errors";
         case ErrorEndOfFile: return "end of file";
         case ErrorIsDir: return "is directory";
+        case ErrorUnsupportedOperatingSystem: return "unsupported operating system";
     }
     return "(invalid error)";
 }