Set CYPRESS_INSTALL_BINARY=0 to avoid installing the Cypress binary

Created by: chrismwendt

Sometimes installing the Cypress binary fails, and causes builds to hang and get backed up

• https://buildkite.com/sourcegraph/sourcegraph/builds/22852#3de607df-0171-4394-a5cc-631a6d564c04
• https://buildkite.com/sourcegraph/sourcegraph/builds/22791#a3351b96-ca36-46b2-8797-4be37018d734 (suspected)

The Cypress e2e tests are currently not being run due to flakiness. See https://github.com/sourcegraph/sourcegraph/pull/700 https://github.com/sourcegraph/sourcegraph/issues/697

This PR does not need to update the CHANGELOG because it's build-related.