Auto merge of #8022 - illicitonion:trywithout, r=ehuss

Try installing exact versions before updating

When an exact version is being installed, if we already have that
version from the index, we don't need to update the index before
installing it. Don't do this if it's not an exact version, because the
update may find us a newer version.

This is particularly useful for scripts which unconditionally run
`cargo install some-crate --version=1.2.3`. Before install-update, I
wrote a crate to do this
(https://crates.io/crates/cargo-ensure-installed) which I'm trying to
replace with just `cargo install`, but the extra latency of updating the
index for a no-op is noticeable.

Author: bors