Usage of cargo's target-dir not consistent with cargo for relative paths

[dhardy]

My Cargo config looks like this:

$ cd $HOME
$ cat .cargo/config 
[build]
target-dir = ".cache/cargo"

Cargo resolves this to $HOME/.cache/cargo. Interpretation of relative paths within config files is detailed here.

When using RLS, I get the following within my project directory:

$ ls .cache/cargo/
CACHEDIR.TAG  rls

RLS's interpretation of this is here.

Related: #1167 #527