Rollup merge of #148224 - jyn514:doc-open, r=Kobzol

bootstrap: `ensure(doc::Std)` no longer opens a browser

In general, the rationale for `--open` is to only open HTML files if they were "explicitly" invoked from the CLI (e.g. `x doc --open library/core`). The existing logic did not do that. Instead it opened the docs unconditionally when a subset of the crates was requested. This is unfortunate for other Steps in bootstrap, which may wish to `ensure()` the standard library docs without opening them.

Change `Std` to check if it was explicitly invoked, rather than assuming it's the case.

Author: matthiaskrgr