jenkins: Temporary workaround for pytest-dev/pytest#920

Fixture collection is not deterministic in the presence of hash
randomisation, so seed the hash so that we can run tests in parallel.

Author: wence-

Jenkinsfile

@@ -6,6 +6,7 @@ pipeline {
     PATH = "/usr/local/bin:/usr/bin:/bin"
     CC = "mpicc"
     FIREDRAKE_CI_TESTS = "1"
+    PYTHONHASHSEED = "12453221"
   }
   stages {
     stage('Clean') {