[bazel][ci] Add CI action to generate pregen patch (#8816)

There were complaints about no patch files being created from CI when
the examples pre-gen fails for people who don't build with bazel. This
adds a new action step to run just the non-robotpy pregen.

I also added an argument to the diff tests to make it a unified diff,
which might make it easier to hand fix.

commandsv3/BUILD.bazel

@@ -28,6 +28,7 @@ generate_commandsv3(
 
 write_source_files(
     name = "write_commandsv3",
+    diff_args = ["-u"],
     files = {
         "src/generated": ":generate_commandsv3",
     },