Return linux as host OS from getConfigString when using musl

linux_musl is an alternative host OS, return "linux" from
getConfigString when it is being used.

Bug: 259266326
Test: build/bazel/ci/mixed_libc.sh
Test: USE_HOST_MUSL=true build/bazel/ci/mixed_libc.sh
Change-Id: I47b8fdd93b36345f82dd4e8455edb4c579f805e9