vim currently doesn't build on cross (for armv6l or aarch64 anyway),
complaining about unable to test what tgetent does in special circumstances.

Since this is a mentioned issue in the vim installation documentation,
this commit adds it to the set of variables we set/override when
cross-compiling.

This fixes cross-compilation of vim on the platforms mentioned above!

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

This is one of the variables mentioned to set
when cross-compiling:

https://github.com/vim/vim/blob/master/src/INSTALLx.txt