use a named pipe for isolated secret passthrough (not stdin)

it's not unusual for the secret data we pass into the `run_isolated.yml`
playbook to be quite long, namely because it can contain RSA key
data; by passing this value into the ansible-playbook process using
`vars_prompt`, we're limited by pexpect's tty line limit (which looks
like it caps out around 4k).  Because of this, large payloads are
being truncated and causing job run failures.

this changes the implementation to use a named pipe instead, which
doesn't have the same limitation

see: #7183

awx/playbooks/run_isolated.yml

@@ -8,10 +8,8 @@
 - name: Prepare data, dispatch job in isolated environment.
   hosts: all
   gather_facts: false
-  vars_prompt:
-    - prompt: "Secret"
-      name: "secret"
-      private: yes
+  vars:
+      secret: "{{ lookup('pipe', 'cat ' + src + '/env') }}"
 
   tasks: