How to pass environment variable received from GitHub actions

I think you are trying to read the wrong environment variable name. GitHub Actions adds INPUT_ to the name of the input variable. So try the following:

print(os.getenv('INPUT_TEST_VAR'))

From the documentation:

When you specify an input to an action in a workflow file or use a default input value, GitHub creates an environment variable for the input with the name INPUT_. The environment variable created converts input names to uppercase letters and replaces spaces with _ characters.

For example, if a workflow defined the numOctocats and octocatEyeColor inputs, the action code could read the values of the inputs using the INPUT_NUMOCTOCATS and INPUT_OCTOCATEYECOLOR environment variables.

https://help.github.com/en/articles/metadata-syntax-for-github-actions#inputs

Keep env vars secret by specifying them in Settings -> Secrets in the repo, and then calling them in the workflow:

For example, consider a workflow that runs an R script followed by a Python script. First, in .github/workflows/my_job.yml notice the MY_VAR variable, which points to a stored secret with ${{ secrets.MY_VAR}}. The rest is standard code (run on cron, specify Ubuntu OS and Docker image, define workflow steps).

on:
  schedule:
    - cron:  '0 17 * * *'
jobs:
  my_job:
    name: my job
    env:
      MY_VAR: ${{ secrets.MY_VAR }}
    runs-on: ubuntu-18.04
    container:
     image: docker.io/my_username/my_image:my_tag
    steps:
      - name: checkout_repo
        uses: actions/checkout@v2
      - name: run some code
        run: bash ./src/run.sh 

Next, in the scripts that compose your workflow, you can access the env var specified in the workflow file above as you would locally.

For example, in the repo, let's assume src/run.sh calls an R script followed by a Python script.

In R access the env var and store as an object:

my_var <- Sys.getenv("MY_VAR")

.
.
.

In Python access the env var and store as an object:

import os

my_var = os.getenv("MY_VAR")
.
.
.

See the docs here.

A bit late but for the next one, you can also use the env field :

name: CI

on: [push]

jobs:
  build:

runs-on: ubuntu-latest

steps:
  - name: Test the GH action
    uses: paramt/github-actions-playground@master
    env:
      test_var: "this is just a test"

which will be included during the creation of your docker and pass without the prefix INPUT_