Add PHP Matrix and caching

[dmason30]

Its common to do a matrix for supported PHP versions and to cache composer dependencies for faster runs.

Note: Installing nova will fail on PRs as it tries to install nova when access to secrets is blocked. The same problem we had with travis.

[mikebronner]

Thanks for the updates :). Blarg, so it looks like actions don't apply env vars to PRs either ... I'll research that more this weekend.

[dmason30]

@mikebronner Yeah you will probs have to go down a similar approach

- name: Remove Nova on a Pull Request
  if: github.event_name == 'pull_request'
  run: composer remove laravel/nova --no-update --no-interaction

# Install dependencies & other steps...

- name: Execute Integration and Feature tests via PHPUnit
  run: vendor/bin/phpunit --configuration phpunit.xml.dist --testsuite Integration,Feature

- name: Execute Nova tests via PHPUnit
  if: github.event_name != 'pull_request'
  run: vendor/bin/phpunit --configuration phpunit.xml.dist --testsuite Nova