lib: workon_helper: replace shell=True usage

Don't force people to use a shell command with our APIs.

BUG=None
TEST=CQ passes

Change-Id: If9091d161fd085b0f4daa3561ba73d02beb78057
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4757753
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/cros_workon.py b/scripts/cros_workon.py
index bece65b..a1b3356 100644
--- a/scripts/cros_workon.py
+++ b/scripts/cros_workon.py
@@ -129,7 +129,7 @@
         elif options.command == "iterate":
             helper.RunCommandInPackages(
                 options.packages,
-                options.iterate_command,
+                ["bash", "-c", options.iterate_command],
                 use_all=options.all,
                 use_workon_only=options.workon_only,
             )