* upgrade to @percy/cypress 3.x

@percy/cypress 2.x uses deprecated @percy/agent which includes
an old version of puppeteer. The old version of puppeteer causes
`yarn` command to fail on M1 macOS environment.

Upgrade to @percy/cypress 3.x removes the dependency and thus
solves the issue.

The upgrade was done by running `npx @percy/migrate`.

* remove percy healthcheck task

This no longer exists in 3.x, see
https://github.com/percy/percy-cypress/blob/master/README.md#upgrading

* remove deprecated percy/exec-action

and moved `percy exec` to the correct command

* run 'yarn install' before running percy