[infra] All build directories are gitignored

[dcharkes]

#1383 is slightly too agresssive on gitignores:

# Build directories are produced when building using the Flutter CLI.
build

We have directories named build inside other places than the toplevel package dir. These contain code/tests/examples.

This blocks:

• [native_assets_cli] Publish 0.7.3 #1494

  * 48 checked-in files are ignored by a `.gitignore`.
    Previous versions of Pub would include those in the published package.