I am very glad to see you here. According to your scenario, I would suggest you use including configurations in a matrix build.

 You can set os as matrix and then use include keyword to specify TARGET, COMPILER and LINKER. In steps, you can use ${{ matrix.TARGET }} to get the value. Please check the example, it will create three jobs::

name: test matrix
on: push
jobs:
  build:
    runs-on: ${{ matrix.os }}
    strategy:
      matrix:
        os: [macos-latest, ubuntu-latest, ubuntu-latest]
        include:
        - os: macos-latest
          TARGET: x86_64-apple-darwin
          COMPILER: clang
          LINKER: clang
    - os: ubuntu-latest
      TARGET: armv7-unknown-linux-musleabihf
      COMPILER: arm-linux-gnueabihf-gcc-5
      LINKER: gcc-5-arm-linux-gnueabihf

    - os: ubuntu-latest
      TARGET: x86_64-unknown-linux-musl
      COMPILER: gcc
      LINKER: gcc
steps:
 - uses: actions/checkout@v1
 - run: echo ${{matrix.TARGET}}

matrix result.png751×315 16.8 KB

For matrix syntax, please refer to this official document:  https://help.github.com/en/actions/automating-your-workflow-with-github-actions/workflow-syntax-for-github-actions#example-including-configurations-in-a-matrix-build

Thank you very much @yanjingzhu 

Just tested this configuration and have to say it does not work. Take a look at screenshots:

Screenshot 2020-10-09 1627251009×342 20.3 KB

Screenshot 2020-10-09 1627061111×349 20.8 KB

Both jobs are x86_64-unknown-linux-musl

As far as I’ve seen in a little experiment on my own, using the same key multiple times under include (here: ubuntu-latest) overwrites the job configuration and the last entry wins. That is, the matrix is configured for 1x macOS and 2x Ubuntu, but first Ubuntu configuration is overwritten by the second, so both end up with TARGET: x86_64-unknown-linux-musl etc.

@sergeyklay’s solution to use include only and no other keys under matrix is the best option IMO. Alternatively, one could use a unique key, but this solution doesn’t add anything over using include only:

build: [1, 2, 3]
include:
  - build: 1
    os: macos-latest
    ...


build: 2

os: ubuntu-latest

...


build: 3

os: ubuntu-latest

...