bootstrap: Use cargo's `build.warnings=deny` rather than -Dwarnings

This has two major advantages. First, it makes us less dependent on the rustc shim, which is nice but not super important. More importantly, it gives us much nicer caching properties.

Previously, `x build --warnings warn && x build --warnings deny` would rebuild all of bootstrap, and if there were any warnings emitted on the last build of the compiler, they would *not* fail the build, because cargo would cache the output rather than rerunning the shim.

After this change, bootstrap rebuilds instantly, and cargo knows that it should fail the build but *without* invalidating the cache.

<details><summary>An example of rebuilding bootstrap after a switch from warn->deny:</summary>

```
INFO: Downloading and building bootstrap before processing --help command.
      See src/bootstrap/README.md for help with common commands.
Building bootstrap
   Compiling bootstrap v0.0.0 (/Users/jyn/src/ferrocene3/src/bootstrap)
warning: unused variable: `x`
    --> src/bootstrap/src/core/builder/mod.rs:1792:13
     |
1792 |         let x: ();
     |             ^
     |
     = note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default
help: if this is intentional, prefix it with an underscore
     |
1792 |         let _x: ();
     |             +
help: you might have meant to pattern match on the similarly named constant `_`
     |
1792 -         let x: ();
1792 +         let utils::render_tests::_: ();
     |

warning: `bootstrap` (lib) generated 1 warning (run `cargo fix --lib -p bootstrap` to apply 1 suggestion)
    Finished `dev` profile [unoptimized] target(s) in 5.14s
error: warnings are denied by `build.warnings` configuration
failed to run: /Users/jyn/src/ferrocene3/build/aarch64-apple-darwin/stage0/bin/cargo build --jobs=default --manifest-path /Users/jyn/src/ferrocene3/src/bootstrap/Cargo.toml -Zroot-dir=/Users/jyn/src/ferrocene3 -Zwarnings
```

</details>

building the compiler from scratch with `deny`: https://gist.github.com/jyn514/493ed26c2aa6f61bf47c21e75efa2175

<details><summary>and rebuilding the compiler after switching from deny->warn (note that it reuses the whole cache, there are no invalidations):</summary>

```
$ x c compiler
Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.15s
Checking stage1 compiler artifacts{rustc-main, rustc_abi, rustc_arena, rustc_ast, rustc_ast_ir, rustc_ast_lowering, rustc_ast_passes, rustc_ast_pretty, rustc_attr_parsing, rustc_baked_icu_data, rustc_borrowck, rustc_builtin_macros, rustc_codegen_llvm, rustc_codegen_ssa, rustc_const_eval, rustc_data_structures, rustc_driver, rustc_driver_impl, rustc_error_codes, rustc_error_messages, rustc_errors, rustc_expand, rustc_feature, rustc_fluent_macro, rustc_fs_util, rustc_graphviz, rustc_hashes, rustc_hir, rustc_hir_analysis, rustc_hir_id, rustc_hir_pretty, rustc_hir_typeck, rustc_incremental, rustc_index, rustc_index_macros, rustc_infer, rustc_interface, rustc_lexer, rustc_lint, rustc_lint_defs, rustc_llvm, rustc_log, rustc_macros, rustc_metadata, rustc_middle, rustc_mir_build, rustc_mir_dataflow, rustc_mir_transform, rustc_monomorphize, rustc_next_trait_solver, rustc_parse, rustc_parse_format, rustc_passes, rustc_pattern_analysis, rustc_privacy, rustc_proc_macro, rustc_public, rustc_public_bridge, rustc_query_impl, rustc_query_system, rustc_resolve, rustc_sanitizers, rustc_serialize, rustc_session, rustc_span, rustc_symbol_mangling, rustc_target, rustc_thread_pool, rustc_trait_selection, rustc_traits, rustc_transmute, rustc_ty_utils, rustc_type_ir, rustc_type_ir_macros, rustc_windows_rc} (stage0 -> stage1, aarch64-apple-darwin)
warning: function `foo` is never used
  --> compiler/rustc_hashes/src/lib.rs:16:4
   |
16 | fn foo() {}
   |    ^^^
   |
   = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default

warning: `rustc_hashes` (lib) generated 1 warning
    Finished `release` profile [optimized + debuginfo] target(s) in 0.53s
Build completed successfully in 0:00:08
```

</details>

thanks to `@epage` for the help finding this!

r? bootstrap

bootstrap: Use cargo's `build.warnings=deny` rather than -Dwarnings

try-job: test-various