Add #![feature(rustc_private)] to driver's main.rs

[smoelius]

Recently, this seems to have become necessary to build drivers on macOS.

[smoelius]

TODO: Find the specific commit that made this PR necessary.

[smoelius]

On macOS, the driver_builder::test::nightly test fails when this line is change to nightly-2024-08-12 (commit 41dd149fd):

dylint/dylint/src/driver_builder.rs

Line 235 in ba0b0ea

"nightly",

But the test passes when the line is changed to nightly-2024-08-11 (commit 730d5d409).

So the relevant commit is somewhere between 730d5d409 and 41dd149fd.

Here is the list of all such commits:

41dd149fd6a Auto merge of #128677 - nikic:llvm19rc2, r=cuviper
9cb1998ea15 Auto merge of #122362 - Zoxc:rustc_driver_static_std, r=oli-obk,lqd,bjorn3,Kobzol
5e5ec8af1b3 Auto merge of #128916 - jieyouxu:dump-ice-dump-ice, r=compiler-errors
6dc300ba453 tests: ignore `dump-ice-to-disk` on windows
c9bd03cb724 Auto merge of #128959 - matthiaskrgr:rollup-6jdqi3l, r=matthiaskrgr
2c88eb9805c Rollup merge of #128882 - RalfJung:local-waker-will-wake, r=cuviper
eff9120b7c5 Rollup merge of #128875 - bvanjoi:cleanup-import-used, r=petrochenkov
32e0fe129d4 Rollup merge of #128762 - fmease:use-more-slice-pats, r=compiler-errors
bd7075c69e7 Rollup merge of #128592 - evelynharthbrooke:master, r=Mark-Simulacrum
853255e28d6 Rollup merge of #128536 - Zalathar:print-cleanup, r=Nadrieril
e8f6819db7e Rollup merge of #120314 - mina86:i, r=Mark-Simulacrum
b22253c4548 Keep rustc's std copy
960e7b55e1b tests: tidy up `dump-ice-to-disk` and make assertion failures extremely verbose
dd3f7578ee5 Exclude just `std` from rustc deps
56beb1d744c Update the unstable book
486864f2359 Don't statically link `std` into `rustc_driver` for `windows-gnu`
736a249954a Ask the user to use `feature(rustc_private)` when linking to `rustc_driver`
3ee43259ac9 Link `std` statically in `rustc_driver`
730d5d4095a Auto merge of #128572 - compiler-errors:fix-elaborate-box-derefs-on-debug, r=saethlin

[smoelius]

This appears to be the relevant PR, though I don't know which commit specifically: rust-lang/rust#122362