Doc tests are not run

[gnzlbg]

Currently, cross test does not run documentation tests.

[Alexhuszagh]

This still seems to depend on a nightly compiler, as documented in doctest-xcompile

• Tracking issue for cross-compiling doc tests (doctest-xcompile) rust-lang/cargo#7040
• Tracking Issue for Cross Compiling Doctests rust-lang/rust#64245

If the subcommand is test and the compiler is nightly, we may be able to provide the -Zdoctest-xcompile flag.