testing: use posborne/rust-cross:x86 for x86_64/i686
testing: use posborne/rust-cross:x86 for x86_64/i686

Previously, std for x86_64 and i686 was included in the base
image.  To reduce size, those contents are now in the x86
image instead.

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

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