fix: matrix loops should be deterministic

[pd93]

I noticed that the unit tests for the matrix implementation are flaky. This is because the order of the input maps is not preserved. e.g.

  loop-matrix:
    cmds:
      - for:
          matrix:
            OS: ["windows", "linux", "darwin"]
            ARCH: ["amd64", "arm64"]
        cmd: echo "{{.ITEM.OS}}/{{.ITEM.ARCH}}"

can result in both:

windows/amd64
windows/arm64
linux/amd64
linux/arm64
darwin/amd64
darwin/arm64

and:

windows/amd64
linux/amd64
darwin/amd64
windows/arm64
linux/arm64
darwin/arm64

I have made it so that the output is always the former example.

[andreynering]

Good catch!