adjust rust-analyzer build directory

[tshepang]

rust-lang/rust#132794 seems to have modified the behavior and path for Helix's build directory, so I think this needs to be updated slightly?

Originally posted by @connortsui20 in #2106 (comment)

[tshepang]

rust-analyzer settings for the other editors should maybe also be adjusted to be consistent with helix settings

[connortsui20]

rust-analyzer settings for the other editors should maybe also be adjusted to be consistent with helix settings

I'm not actually sure what the difference is between the settings but as someone who likes to use multiple editors depending on the task maybe it could be possible to ensure that these settings are synced in some way? Either that or just make it consistent from the start.

Should an issue be made in the rust repository itself?

[tshepang]

that, or a pr to make it consistent

[jieyouxu]

I would recommend instead to drop any explicit config examples in rustc-dev-guide, and instead backlink to src/etc/* config files to keep them in async.