CARGO_OPTIONS is empty

[kristof-mattei]

rust-ci-github-actions-workflow/README.md

Line 77 in f86891a

- `$CARGO_OPTIONS` includes `CARGO_INCREMENTAL`, `RUSTFLAGS`, and `RUSTDOCFLAGS` options needed for code coverage.

I've been playing with this awesome template, but I'm having some issues understanding how CARGO_OPTIONS is supposed to be constructed.

From the documentation it should magically contain CARGO_INCREMENTAL, RUSTFLAGS and RUSTDOCFLAGS, but could it be that this join was missed somewhere?

@lumisce for visibility