Fix bug where stdout was checked instead of stderr

Author: daurnimator

lib/std/build/run.zig

@@ -182,7 +182,7 @@ pub const RunStep = struct {
         defer self.builder.allocator.free(stdout);
 
         var stderr: []const u8 = undefined;
-        switch (self.stdout_behavior) {
+        switch (self.stderr_behavior) {
             .expect_exact, .expect_matches => {
                 var stderr_file_in_stream = child.stderr.?.inStream();
                 stderr = stderr_file_in_stream.stream.readAllAlloc(self.builder.allocator, max_stdout_size) catch unreachable;