[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.

wpiutil/BUILD.bazel

@@ -77,6 +77,7 @@ generate_wpiutil(
 
 write_source_files(
     name = "write_wpiutil",
+    diff_args = ["-u"],
     files = {
         "src/generated/test/native/cpp": ":generate_wpiutil",
     },