bug(experimenter): give glean a unique data dir per pid (#13789)

Because

- glean must have a unique data dir per pid

This commit

- Adds a process id subdir to the glean data dir path

Fixes #13788

Author: relud

experimenter/experimenter/settings.py

@@ -10,6 +10,7 @@
 https://docs.djangoproject.com/en/1.9/ref/settings/
 """
 
+import os
 from pathlib import Path
 from urllib.parse import urljoin
 
@@ -74,7 +75,7 @@
 
 GLEAN_UPLOAD_ENABLED = config("GLEAN_UPLOAD_ENABLED", default=False, cast=bool)
 
-GLEAN_DATA_DIR = config("GLEAN_DATA_DIR", default="/var/glean")
+GLEAN_DATA_DIR = Path(config("GLEAN_DATA_DIR", default="/var/glean")) / str(os.getpid())
 
 # Application definition