Load yaml and insert values into other yaml arrays

[atye]

I have this yaml:

- variable: "username"
  value: "myusername"
- variable: "password"
  value: "mypassword"

I want to put the variable values into this yaml:

- !policy
  id: secrets
  body:
    - &variables []
    - !permit
      role: !host /myid/system:serviceaccount:mynamespace:mysa
      privilege: [ read, execute ]
      resource: *variables

so that it ends up looking like:

- !policy
  id: secrets
  body:
    - &variables
       - !variable username
       - !variable password
    - !permit
      role: !host /myid/system:serviceaccount:mynamespace:mysa
      privilege: [ read, execute ]
      resource: *variables

I've been trying various commands but I'll show this one:

podman run --user="root" -v $PWD:/workdir --rm mikefarah/yq:latest 'load("/workdir/creds.yaml")[] as $item | .[] | select(.id == "secrets") | .body[0] += ($item.variable | tag("!variable"))' /workdir/test.yaml
Error: bad expression, please check expression syntax

What is the correct way to do this?