[BUG] Validate flags argument

Describe the bug
We were dynamically generating the flags argument and passing it into flags: on the action using an environment variable. The report showed it was being uploaded. There were no upload processing errors. But codecov was not being triggered.

To Reproduce
Use:

   - name: Publish coverage report
      uses: codecov/codecov-action@v3
      with:
        token: ${{ secrets.CODECOV_TOKEN }}
        flags: $CODECOV
        name: ${{ matrix.name }}
        verbose: false
        fail_ci_if_error: true
      env:
        CODECOV_TOKEN: "${{ secrets.CODECOV_TOKEN }}"

Expected behavior
It should have some kind of warning if it does not support $ in the flags argument,. Instead it makes you think that the report was uploaded successfully and never shows in the UI.

We have since corrected our action to use ${{ env.CODECOV }} however it was not easy for us to track down.

Start by examining how the action reads and validates the flags input, then reproduce the provided workflow using $CODECOV and ${{ env.CODECOV }}. Done means unsupported shell-style variable syntax produces a clear warning instead of appearing successful without triggering Codecov processing.