Explicitly set pip cache dir for pip builds.
Explicitly set pip cache dir for pip builds.

- the default cache location on github actions is inaccessible resulting in caching being disabled and builds taking a long time.

cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci