Fix: don't purge pip's cache which apparently is empty

4 jobs for master in 73 minutes and 34 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #363506
build-image

01:06:55

 
  Test
passed #363509
tests_cpp_packages

00:05:17

passed #363507
tests_python_packages

00:05:15

passed #363508
tests_R_packages

00:06:38