[ci-workers] Install Python modules in a virtualenv

contrib/ci-workers/vars/Archlinux.yml

@@ -4,4 +4,3 @@ buildbot_deps:
 build_deps:
   - multilib/gcc
   - make
-pip_bin_dir: /usr/bin

contrib/ci-workers/vars/CentOS.yml

@@ -10,4 +10,3 @@ dist_deps:
   - pkgconfig # Required until b556beda264308e040f8d88aca4f2f386a0183d9 is pulled in
   - python-devel
   - redhat-rpm-config
-pip_bin_dir: /usr/bin

contrib/ci-workers/vars/default.yml

@@ -34,6 +34,10 @@ grind_deps:
   - lcov
   - valgrind
 
+# Python modules required on the system
+system_modules:
+  - virtualenv
+
 # Python modules required for a Zcash Buildbot worker
 buildbot_modules:
   - pip # Needs to be updated first so Buildbot installs
@@ -44,6 +48,3 @@ buildbot_modules:
 rpc_test_modules:
   - pyblake2
   - pyzmq
-
-# Environment variables
-pip_bin_dir: /usr/local/bin