Sets the `sysroot` field in rust-project.json which is now needed in
newer versions of rust-analyzer instead of the `sysroot_src` field.

Till [1] `rust-analyzer` used to guess the `sysroot` based on the
`sysroot_src` at [2]. Now `sysroot` is a required parameter for a
`rust-project.json` file. It is required because `rust-analyzer`
need it to find the proc-macro server [3].

In the current version of `rust-analyzer` the `sysroot_src` is only used
to include the inbuilt library crates (std, core, alloc, etc) [4]. Since
we already specify the core library to be included in the
`rust-project.json` we don't need to define the `sysroot_src`.

Code editors like VS Code try to use the latest version of rust-analyzer
(which is updated every week) instead of the version of rust-analyzer
that comes with the rustup toolchain (which is updated every six weeks
along with the rust version).

Without this change `rust-analyzer` is breaking for anyone using VS Code.
As they are getting the latest version of `rust-analyzer` with the
changes made in [1].

`rust-analyzer` will also start breaking for other developers as they
update their rust version (assuming that also updates the rust-analyzer
version on their system).

This patch should work with every setup as there is no more guess work
being done by `rust-analyzer`.

[ Lukas, who leads the rust-analyzer team, says:

    `sysroot_src` is required now if you want to have the sysroot
    source libraries be loaded. I think we used to infer it as
    `{sysroot}/lib/rustlib/src/rust/library` before when only the
    `sysroot` field was given but that was since changed to make it
    possible in having a sysroot without the standard library sources
    (that is only have the binaries available). So if you want the
    library sources to be loaded by rust-analyzer you will have to set
    that field as well now.

  - Miguel ]

Link: https://github.com/rust-lang/rust-analyzer/pull/17287 [1]
Link: https://github.com/rust-lang/rust-analyzer/blob/f372a8a1176ff8dd5f45ab2ddd45f3530db0374f/crates/project-model/src/workspace.rs#L367-L374 [2]
Link: https://github.com/rust-lang/rust-analyzer/blob/eeb192b79aeac47b40add66347022af17a74fbaf/crates/project-model/src/sysroot.rs#L180-L192 [3]
Link: https://github.com/search?q=repo%3AVeykril%2Frust-analyzer%20src_root()&type=code [4]
Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Sarthak Singh <sarthak.singh99@gmail.com>
Link: https://rust-for-linux.zulipchat.com/#narrow/stream/291565-Help/topic/How.20to.20rust-analyzer.20correctly.20working
Link: https://lore.kernel.org/r/20240724172713.899399-1-sarthak.singh99@gmail.com
[ Formatted comment, fixed typo and removed spurious empty line. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>