Document CYPRESS_BINARY_VERSION environment variable

[bahmutov]

Cypress installer can override binary version and download a different binary from version specified in package.json. For that installer checks CYPRESS_BINARY_VERSION environment variable.

For example you can install specific version

CYPRESS_BINARY_VERSION=0.20.0 npm install cypress

In addition, once https://github.com/cypress-io/cypress-monorepo/pull/536 lands you can specify full url too

CYPRESS_BINARY_VERSION=https://my-server/foo/cypress.zip npm install cypress

[bahmutov]

Would like to implement version + file + url first cypress-io/cypress#701 and then document all three ways