Make stylelint ignore the build/ directory
Make stylelint ignore the build/ directory

CQ has started failing because now the `build/` dir seems to contain CSS
files that it's linting
(https://ci.chromium.org/ui/p/devtools-frontend/builders/try/devtools_frontend_linux_rel/b8834935955252908353/overview).

Bug: None
Change-Id: I310159cfbb79ed7653ea808d1f67fc3bf0683729
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3190270
Commit-Queue: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>
Commit-Queue: Wolfgang Beyer <573aa99bb19da47d816981118f95c945310dd3f4@chromium.org>
Auto-Submit: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>
Reviewed-by: Wolfgang Beyer <573aa99bb19da47d816981118f95c945310dd3f4@chromium.org>

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