Force skipping a component when installing marks it as installed

[Nemo157]

Problem
When force skipping a component during install a later attempt to add that component will act as if the component is installed.

Steps

> rustup toolchain install nightly-2019-10-16 --force
info: syncing channel updates for 'nightly-2019-10-16-x86_64-unknown-linux-gnu'
info: latest update on 2019-10-16, rust version 1.40.0-nightly (237d54ff6 2019-10-15)
warning: Force-skipping unavailable component 'clippy-x86_64-unknown-linux-gnu'
[...]

  nightly-2019-10-16-x86_64-unknown-linux-gnu installed - rustc 1.40.0-nightly (237d54ff6 2019-10-15)

> rustup component add clippy-preview --toolchain nightly-2019-10-16
info: component 'clippy' for target 'x86_64-unknown-linux-gnu' is up to date
> cargo +nightly-2019-10-16 clippy --version
error: 'cargo-clippy' is not installed for the toolchain 'nightly-2019-10-16-x86_64-unknown-linux-gnu'
To install, run `rustup component add clippy --toolchain nightly-2019-10-16-x86_64-unknown-linux-gnu`

Notes

Output of rustup --version: rustup 1.20.1 (4eadfff3d 2019-10-15)