Build failure repro command may include --features flag without any features

[obi1kenobi]

Note that the repro command below has an empty list of features after the --features flag:

note: the following command can be used to reproduce the compilation error:
      cargo new --lib example &&
          cd example &&
          echo '[workspace]' >> Cargo.toml &&
          cargo add --path /tmp/sequoia-cert-store --no-default-features --features  &&
          cargo check

This should be fixed.

Surfaced in #717.