ci: only attempt travis-cargo under linux
ci: only attempt travis-cargo under linux

Signed-off-by: Paul Osborne <b6afffd6c1a00645fdd04b09edf63d034a949a39@gmail.com>

geofft/nix-rust,MarkusJais/nix-rust,berkowski/nix,berkowski/nix,xd009642/nix,nix-rust/nix,Susurrus/nix,kamalmarhubi/nix-rust,kamalmarhubi/nix-rust,Susurrus/nix,MarkusJais/nix-rust,MarkusJais/nix-rust,Susurrus/nix,asomers/nix,geofft/nix-rust,carllerche/nix-rust,posborne/rust-nix,xd009642/nix,geofft/nix-rust,posborne/rust-nix,asomers/nix,carllerche/nix-rust,xd009642/nix,kamalmarhubi/nix-rust,carllerche/nix-rust,nix-rust/nix,berkowski/nix,posborne/rust-nix