INPUT_* environment variables are missing in composite actions

According to the documentation actions should receive their input values as environment variables prefixed with INPUT_ in addition to the inputs context. These environment variables are missing for composite actions.

To Reproduce
Steps to reproduce the behavior:

1. Create a composite action with one or more inputs, and a step that outputs the associated INPUT variables:

inputs:
  foo:
    description: 'Test input'
    required: yes
runs:
  using: composite
  steps:
    - run: |
        echo "FOO: ${INPUT_FOO}"
      shell: bash

2. Use the action in a workflow:

steps:
  - uses: ./example-action
    with:
      foo: 'bar'

3. Observe the lack of value after the FOO: prefix.

I have an example action and workflow (the uses: ./composite-action step) that demonstrate the issue and the obvious workaround. There's a workflow run as well (check the "Run /./composite-action" step).

Expected behavior
Steps should be able to read inputs via environment variables, INPUT_FOO in the example above, INPUT_NUM1 (to 4) in my reproducer.

Runner Version and Platform

Version of your runner?

• 2.272.0

OS of the machine running the runner? OSX/Windows/Linux/...

• Ubuntu 20.04.1 LTS (Github hosted runner VM)

Job Log Output

From the workflow run linked above:

Run ./composite-action
  with:
    num1: 1
    num2: 4
    num3: 0
    num4: 0
Show INPUT vars
  INPUT_NUM1 = 
  INPUT_NUM2 = 
  INPUT_NUM3 = 
  INPUT_NUM4 = 
Add numbers

Credit

I became aware of the issue thanks to a post on the Github community forum and tried to reproduce the issue out of curiosity.

Start with the reproducer files composite-action/action.yaml and .github/workflows/main.yml, then run the workflow and inspect how composite-action inputs are handled by the runner. Done means the example's steps receive INPUT_NUM1 through INPUT_NUM4 with their configured values, while the existing input context continues to work.