GAX Showcase tests now take 40 minutes to run

[bshaffer]

Since merging #9299 , the Showcase Conformance tests have gone from 5m to 40m. I believe this has to do with gRPC being required in the setup, and that it isn't being pulled from cache as it is for our other tests.

https://github.com/googleapis/google-cloud-php/actions/workflows/conformance-tests-gax-showcase.yaml

This should be an easy fix, we just need to configure it similarly to our unit tests to use the same extensions cache:

google-cloud-php/.github/workflows/unit-tests.yaml

Lines 37 to 43 in 06825ce

	- name: Setup cache environment
	id: extcache
	uses: shivammathur/cache-extensions@v1
	with:
	php-version: ${{ matrix.php }}
	extensions: sodium, sysvshm, ${{ matrix.extensions }}
	key: cache-key-1 # increment to bust the cache