Github Workflows: Changing or specifying environment variables in matrix jobs - yaml

Question

How do I specify or change an environment variable as part of a job matrix?
For example, here I want my job to echo "Two" and "Three" instead of "One", but github completely ignores the definition or change of the environment variable in the matrix:
name: test-test-test
on:
push:
branches: [ main, develop ]
pull_request:
branches: [ main, develop ]
env:
MY_VAR: One
jobs:
test:
runs-on: ubuntu-latest
strategy:
matrix:
include:
- env:
MY_VAR: Two
- env:
MY_VAR: Three
steps:
- run: echo "$MY_VAR2"
yields echo "One" and echo "One" in both jobs.

Answer 1

Use this context to access information about environment variables that have been set in a workflow, step, or job. Note that you cannot use the env context in the value of the id and uses keys within a step. An example is env.env_var.
GitHub provides extensive documentation about contexts at
https://docs.github.com/en/actions/learn-github-actions/contexts.

Answer 2

The syntax you used won't work for matrix. As stated in the official documentation:
The variables that you define become properties in the matrix context, and you can reference the property in other areas of your workflow file.
In your case, to access the matrix env variable you set in the include field list, you would need to use ${{ matrix.env.MY_VAR }}.
Something like this:
env:
MY_VAR: One
jobs:
test:
runs-on: ubuntu-latest
strategy:
matrix:
include:
- env:
MY_VAR: Two
- env:
MY_VAR: Three
steps:
- run: |
echo "$MY_VAR" # is equal to ${{ env.MY_VAR }}
echo ${{ matrix.env.MY_VAR }}
This will generate 2 test jobs: test (Two) and test (Three).
I made a test here if you want to have a look.
Conclusion:
Basically, the env you set at the workflow level isn't the same env set in the matrix strategy, so you can't substitute the value there.
Alternative
What you could do eventually (as #Adam stated in the other answer) is to set the env value at the workflow level for it to work, in the environment context. (Reference in the documentation + another workflow example).
Example:
env:
WORKFLOW_VARIABLE: WORKFLOW
jobs:
test:
runs-on: ubuntu-latest
env:
JOB_VARIABLE: JOB
steps:
- name: Example
env:
STEP_VARIABLE: STEP
run: |
echo "Hello World"
echo "This is the $WORKFLOW_VARIABLE environment variable"
echo "This is the $JOB_VARIABLE environment variable"
echo "This is the $STEP_VARIABLE environment variable"
But depending on what you plan to achieve, a matrix strategy you be more efficient.