Rename `matrix.os` to `matrix.runner` #155

Job	Run time
build-test (ubuntu-latest, gcc, amd64)	19m 51s
build-test (ubuntu-latest, gcc, c)	25m 34s
build-test (ubuntu-latest, clang, amd64)	21m 56s
build-test (ubuntu-latest, clang, c)	29m 28s
build-test (ubuntu-latest, clang, llvm)	31m 20s
build-test (macos-13, clang, amd64)	45m 27s
build-test (macos-13, clang, c)	50m 55s
build-test (macos-13, clang, llvm)	53m 6s
build-test (macos-14, clang, c)	28s
build-test (macos-14, clang, llvm)	59s
build-test (windows-latest, gcc, amd64)	42m 50s
	5h 21m 54s

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