Revert "Make doc CI use nightly (#16147)" (#16891)

This reverts commit 58a7392.

# Objective

This is causing local `cargo run -p ci` runs to fail on `stable` Rust
when used locally. Fixes #16871.

## Solution

Revert #16147. We shouldn't have merged this until we had a full
solution (I missed a Controversial tag, sorry!).

## Testing

`cargo run -p ci` fails for me before this change in the reported
fashion. After this reversion, it passes.

.github/workflows/ci.yml

@@ -126,7 +126,6 @@ jobs:
       - name: Check Compile
         # See tools/ci/src/main.rs for the commands this runs
         run: cargo run -p ci -- compile
-
   check-compiles-no-std:
     runs-on: ubuntu-latest
     timeout-minutes: 30
@@ -295,7 +294,6 @@ jobs:
         with:
           name: example-run-macos
           path: example-run/
-
   check-doc:
     runs-on: ubuntu-latest
     timeout-minutes: 30
@@ -310,9 +308,7 @@ jobs:
             ~/.cargo/git/db/
             target/
           key: ${{ runner.os }}-check-doc-${{ hashFiles('**/Cargo.toml') }}
-      - uses: dtolnay/rust-toolchain@master
-        with:
-          toolchain: ${{ env.NIGHTLY_TOOLCHAIN }}
+      - uses: dtolnay/rust-toolchain@stable
       - name: Install Linux dependencies
         uses: ./.github/actions/install-linux-deps
         with:
@@ -323,7 +319,7 @@ jobs:
         run: cargo run -p ci -- doc
         env:
           CARGO_INCREMENTAL: 0
-          RUSTFLAGS: "-C debuginfo=0 --cfg docsrs_dep"
+          RUSTFLAGS: "-C debuginfo=0"
       # This currently report a lot of false positives
       # Enable it again once it's fixed - https://github.com/bevyengine/bevy/issues/1983
       # - name: Installs cargo-deadlinks

tools/ci/src/commands/doc_check.rs

@@ -16,6 +16,6 @@ impl Prepare for DocCheckCommand {
             ),
             "Please fix doc warnings in output above.",
         )
-        .with_env_var("RUSTDOCFLAGS", "-D warnings --cfg=docsrs")]
+        .with_env_var("RUSTDOCFLAGS", "-D warnings")]
     }
 }