Fix CI: root toolchain pin and missing rust-src component

rust-toolchain.toml pinned stable-x86_64-pc-windows-gnu, a local Windows
workaround that leaked into the repo root and broke toolchain resolution
on Linux CI ("target tuple in channel name"). Dropped the host suffix to
match the existing convention in kernel/rust-toolchain.toml and
xtask/rust-toolchain.toml.

kernel-tests and boot jobs invoke xtask, which pulls in the bootloader
crate's build script — it compiles its own bare-metal boot-sector/stage
helper binaries and needs rust-src to build core/alloc from source. Never
surfaced locally because rust-src was already installed on the dev
machine; a fresh CI runner hits it immediately.

Author: jlika25

.github/workflows/ci.yml

@@ -77,7 +77,12 @@ jobs:
         with:
           toolchain: nightly
           targets: x86_64-unknown-none
-          components: llvm-tools
+          # `rust-src`: the `bootloader` crate's build script (invoked via
+          # `xtask` in this job) compiles its own boot-sector/stage-2/3/4
+          # helper binaries for bare-metal targets, which needs to build
+          # core/alloc from source — without this component that fails with
+          # "does not exist, unable to build with the standard library".
+          components: llvm-tools, rust-src
       - uses: Swatinem/rust-cache@v2
         with:
           workspaces: |
@@ -115,7 +120,12 @@ jobs:
         with:
           toolchain: nightly
           targets: x86_64-unknown-none
-          components: llvm-tools
+          # `rust-src`: the `bootloader` crate's build script (invoked via
+          # `xtask` in this job) compiles its own boot-sector/stage-2/3/4
+          # helper binaries for bare-metal targets, which needs to build
+          # core/alloc from source — without this component that fails with
+          # "does not exist, unable to build with the standard library".
+          components: llvm-tools, rust-src
       - uses: Swatinem/rust-cache@v2
         with:
           workspaces: |

rust-toolchain.toml

@@ -1,3 +1,8 @@
 [toolchain]
-channel = "stable-x86_64-pc-windows-gnu"
+# No host suffix on purpose (CI runs on Linux) — same convention as
+# kernel/rust-toolchain.toml and xtask/rust-toolchain.toml. On a Windows dev
+# box with no MSVC Build Tools (link.exe), the host-default stable resolves
+# to -msvc and fails to link — invoke explicitly instead:
+#   rustup run stable-x86_64-pc-windows-gnu cargo build
+channel = "stable"
 components = ["clippy", "rustfmt"]