installer: clarify starting assumption task
installer: clarify starting assumption task

We had a macOS user present in Matrix with some confusion because the
lack of a clear task statement here made them think the error meant
that a problem had occurred during the preceding task in a macOS
install: "Fixing any leftover Nix volume state"

NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix