Fix sizebot (#31535)

Our CI workflows generally cache `**/node_modules` (note the glob, it
caches all transitive node_module directories) to speed up startup for
new jobs that don't change any dependencies. However it seems like one
of our caches got into a weird state (not sure how it happened) where
the `build` directory (used in various other scripts as the directory
for compiled React packages) would contain a `node_modules` directory as
well. This made sizebot size change messages very big since it would try
to compare every single file in `build/node_modules`.

The fix is to ensure we always clean the `build` directory before doing
anything with it. We can also delete that one problematic cache but this
PR is a little more resilient to other weird behavior with that
directory.