Use npmjs.org instead of yarnpkg.com

Created by: nicksnyder

This is an attempt to fix the recurring flakiness that we have been seeing in our builds by switching to npmjs.org

info There appears to be trouble with your network connection. Retrying...
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error An unexpected error occurred: "https://registry.yarnpkg.com/mdi-react/-/mdi-react-4.0.0.tgz: ESOCKETTIMEDOUT".
info If you think this is a bug, please open a bug report with the information provided in "/buildkite/builds/buildkite-agent-797f4d46cb-slr2l-1/sourcegraph/.golang/sourcegraph/src/github.com/sourcegraph/sourcegraph/yarn-error.log".
info Visit https://yarnpkg.com/en/docs/cli/install for documentation about this command.
info There appears to be trouble with your network connection. Retrying...
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see https://github.com/yarnpkg/yarn/issues/2508#issuecomment-283655600