config: flush autogenerated config template

#13774

Flush the buffered writer after writing the template so newly created
configuration files contain the expected guidance instead of being empty.

src/config/Config.zig

@@ -4121,6 +4121,7 @@ fn writeConfigTemplate(path: []const u8) !void {
         @embedFile("./config-template"),
         .{ .path = path },
     );
+    try writer.flush();
 }
 
 /// Load configurations from the default configuration files. The default