[ci] add pip cache to CI script

5 jobs from ci in 2 minutes and 40 seconds
Status Job ID Name Coverage
  Build
passed #876
linux
build:linux

00:01:25

passed #875
macOS
build:mac

00:02:31

 
  Install
passed #878
linux install
install:linux

00:00:04

passed #877
macOS install
install:mac

00:00:05

 
  Test
failed #879
linux
test_spm_full_prep:linux

00:00:02

 
Name Stage Failure
failed
test_spm_full_prep:linux Test
    working_set = WorkingSet._build_master()
File "/home/gitlab-runner/miniconda2/envs/clinica_env/lib/python2.7/site-packages/pkg_resources/__init__.py", line 574, in _build_master
ws.require(__requires__)
File "/home/gitlab-runner/miniconda2/envs/clinica_env/lib/python2.7/site-packages/pkg_resources/__init__.py", line 892, in require
needed = self.resolve(parse_requirements(requirements))
File "/home/gitlab-runner/miniconda2/envs/clinica_env/lib/python2.7/site-packages/pkg_resources/__init__.py", line 778, in resolve
raise DistributionNotFound(req, requirers)
pkg_resources.DistributionNotFound: The 'Clinica' distribution was not found and is required by the application
ERROR: Job failed: exit status 1