When calling the `rust_is_available.sh` script, we need to make the
jobserver available to it, as commit ecab4115 ("kbuild: mark `rustc`
(and others) invocations as recursive") explains and did for the others.

Otherwise, we get a warning from `rustc` when calling `make rust-analyzer`
with parallel jobs, e.g. `-j8`. Using several jobs for that target does
not really matter, but developers may call `make` with jobs enabled in
all cases.

Thus fix it.

Fixes: 6dc9d9ca ("kbuild: rust-analyzer: better error handling")
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240806233559.246705-1-ojeda@kernel.org
[ Reworded to add a couple more details mentioned in the list. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>