Expose `strategy` and `matrix` variables in `context`

[tony-pizza]

Is your feature request related to a problem? Please describe.

I'm using a matrix and I want to access matrix- and strategy-related variables in my github-script. The two ways to do that are:

1. Setting env vars:

# ...
strategy:
  matrix:
    group: [1, 2]
steps:
  - uses: actions/github-script@v7
    env:
      MATRIX_GROUP: ${{ matrix.group }}
      MATRIX_NUM_GROUPS: ${{ strategy.job-total }}
    with:
      script: |
        const group = parseInt(process.env.MATRIX_GROUP)
        const numGroups = parseInt(process.env.MATRIX_NUM_GROUPS)

2. Interpolation:

# ...
strategy:
  matrix:
    group: [1, 2]
steps:
  - uses: actions/github-script@v7
    with:
      script: |
        const group = ${{ matrix.group }}
        const numGroups = ${{ strategy.job-total }}

Describe the solution you'd like It would be nice if these were offered somewhere in the context object, which seems like an intuitive home for them. Something like...

# ...
strategy:
  matrix:
    group: [1, 2]
steps:
  - uses: actions/github-script@v7
    with:
      script: |
        const group = parseInt(context.strategy.matrix.group)
        const numGroups = context.strategy.totalJobs