A much simpler way to set the sysroot. (#34)

## What Changed?

Reimplements #32 in a simpler manner. Partially reverts those changes
but with the same overall outcome.

Also added documentation for what is happening.

## Why Does It Need To?

The other version to achieve this needed more changes to `rustc_plugin`
that were haphazard and would have been annoying to try and get merged
eventually. This way is slightly more hacky but with a *significantly*
reduced footprint. Everything is now contained in `dfpp` alone.

## Checklist

- [x] Above description has been filled out so that upon quash merge we
have a
  good record of what changed.
- [x] New functions, methods, types are documented. Old documentation is
updated
  if necessary
- [x] Documentation in Notion has been updated
- [x] Tests for new behaviors are provided
  - [x] New test suites (if any) ave been added to the CI tests (in
`.github/workflows/rust.yml`) either as compiler test or integration
test.
*Or* justification for their omission from CI has been provided in this
PR
    description.

Cargo.toml

@@ -69,5 +69,5 @@ debug = true
 
 [replace]
 
-"rustc_utils:0.6.0-nightly-2023-04-12" = { git = "https://github.com/JustusAdam/rustc_plugin", rev = "a50617487e129623df946c42d554bd8080765096" }
-"rustc_plugin:0.6.0-nightly-2023-04-12" = { git = "https://github.com/JustusAdam/rustc_plugin", rev = "a50617487e129623df946c42d554bd8080765096" }
+"rustc_utils:0.6.0-nightly-2023-04-12" = { git = "https://github.com/JustusAdam/rustc_plugin", rev = "344ecb3bc056c641eaf9d06792571b3d81c8eb33" }
+"rustc_plugin:0.6.0-nightly-2023-04-12" = { git = "https://github.com/JustusAdam/rustc_plugin", rev = "344ecb3bc056c641eaf9d06792571b3d81c8eb33" }

src/lib.rs

@@ -227,10 +227,19 @@ impl rustc_plugin::RustcPlugin for DfppPlugin {
     ) -> rustc_plugin::RustcPluginArgs<Self::Args> {
         use clap::Parser;
         let args = ArgWrapper::parse();
+
+        // Override the SYSROOT so that it points to the version we were
+        // compiled against.
+        //
+        // This is actually not necessary for *this* binary, but it will bne
+        // inherited by the calls to `cargo` and `rustc` does by `rustc_plugin`
+        // and thus those will link against the version of `std` that we
+        // require.
+        std::env::set_var("SYSROOT", env!("SYSROOT_PATH"));
+
         rustc_plugin::RustcPluginArgs {
             args: Args::from_parseable(args.args).unwrap(),
             filter: CrateFilter::OnlyWorkspace,
-            env: vec![("SYSROOT".into(), env!("SYSROOT_PATH").into())],
         }
     }