Clarify case-(in)sensitivity for `matrix.<keys>`

[jsoref]

Code of Conduct

• I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions
https://docs.github.com/en/actions/learn-github-actions/contexts#matrix-context

What part(s) of the article would you like to see updated?

Add an indication whether or not matrix elements are case sensitive.

Additional information

Some sections indicate if items are case sensitive / case insensitive.

Developers of one project w/ which I have some affiliation were asking whether matrix.<key> would be case sensitive.

Here's a subset of the workflow they had:

jobs:
 linux:
 strategy:
 fail-fast: false
 matrix:
 arch:
 - amd64
 include:
 - arch: amd64
 CC: x86_64-linux-gnu
 - arch: arm64
 CC: aarch64-linux-gnu
 runs-on: ubuntu-22.04
 env:
 AR: ${{ matrix.CC }}-ar
 CHOST: ${{ matrix.cc }}
 CC: ${{ matrix.cc }}-gcc
 CPP: ${{ matrix.cc }}-cpp
 CXX: ${{ matrix.cc }}-g++

[cmwilson21]

@jsoref Thanks for opening an issue and providing the workflow. I'll get this triaged for review 👀

[Bryanbasor]

#https://reyal,metrix`
Engineering Scorecard for Metrix.com could not be generated. Please check the repository name & make sure it is part of github organization `

[jsoref]

@cmwilson21

[nguyenalex836]

Reopened

[AMER88795]

matrix.CC