provision: Use a more efficient approach for getting yarn version.

Since yarn has a package.json conveniently available, we can parse
that with jq, saving the expensive operation of starting up yarn.

This saves ~300ms in a no-op provision.

scripts/lib/setup_venv.py

@@ -37,6 +37,12 @@ VENV_DEPENDENCIES = [
     "libxslt1-dev",         # Used for installing talon
     "libpq-dev",            # Needed by psycopg2
     "libssl-dev",           # Needed to build pycurl and other libraries
+
+    # This is technically a node dependency, but we add it here
+    # because we don't have another place that we install apt packages
+    # on upgrade of a production server, and it's not worth adding
+    # another call to `apt install` for.
+    "jq",                   # Used by scripts/lib/install-node to check yarn version
 ]
 
 codename = parse_lsb_release()["DISTRIB_CODENAME"]