add --legacy-peer-deps for ci due to npm 7 changes

Fixes #3040

This is only a CI change. No library changes. This reverts npm behavior to how it worked in NPM v4-6.

Please complete for every pull request

[ ] First-time contributors should sign the Contributor License Agreement. It's a fancy way of saying that you are giving away your contribution to this project. If you haven't before, wait a few minutes and a bot will comment on this pull request with instructions.
[ ] npm run lint:fix has been run and linting passes.
[ ] Mocha, Jest (Storyshots), and components/component-docs.json CI tests pass (npm test).
[ ] Tests have been added for new props to prevent regressions in the future. See readme.
[ ] Review the appropriate Storybook stories. Open http://localhost:9001/.
[ ] Review tests are passing in the browser. Open http://localhost:8001/.
[ ] Review markup conforms to SLDS by looking at DOM snapshot strings.

[ ] CircleCI tests pass. This includes linting, Mocha, Jest, Storyshots, and components/component-docs.json tests.
[ ] Tests have been added for new props to prevent regressions in the future. See readme.
[ ] Review the appropriate Storybook stories. Open http://localhost:9001/.
[ ] The Accessibility panel of each Storybook story has 0 violations (aXe). Open http://localhost:9001/.
[ ] Review tests are passing in the browser. Open http://localhost:8001/.
[ ] Review markup conforms to SLDS by looking at DOM snapshot strings.
Required only if there are markup / UX changes
[ ] Add year-first date and commit SHA to last-slds-markup-review in package.json and push.
[ ] Request a review of the deployed Heroku app by the Salesforce UX Accessibility Team.
[ ] Add year-first review date, and commit SHA, last-accessibility-review, to package.json and push.
[ ] While the contributor's branch is checked out, run npm run local-update within locally cloned site repo to confirm the site will function correctly at the next release.

salesforce / design-system-react