Remove redundant toolchain info from GH workflow

Unfortunately, [actions-rs/toolchain doesn’t support TOML-formatted
rust-toolchain files](actions-rs/toolchain#126), so this calls
`rustup show` directly where possible.

.github/workflows/ci.yml

@@ -10,18 +10,11 @@ jobs:
   check:
     name: Check
     runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        rust:
-          - stable
     steps:
       - uses: actions/checkout@v4
         with:
           submodules: true
-      - uses: actions-rs/toolchain@v1
-        with:
-          toolchain: ${{ matrix.rust }}
-          override: true
+      - run: rustup show
       - uses: actions-rs/cargo@v1
         with:
           command: check
@@ -111,18 +104,11 @@ jobs:
   fmt:
     name: Rustfmt
     runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        rust:
-          - stable
     steps:
       - uses: actions/checkout@v4
         with:
           submodules: true
-      - uses: actions-rs/toolchain@v1
-        with:
-          toolchain: ${{ matrix.rust }}
-          override: true
+      - run: rustup show
       - run: rustup component add rustfmt
       - uses: actions-rs/cargo@v1
         with:
@@ -132,18 +118,11 @@ jobs:
   clippy:
     name: Clippy
     runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        rust:
-          - stable
     steps:
       - uses: actions/checkout@v4
         with:
           submodules: true
-      - uses: actions-rs/toolchain@v1
-        with:
-          toolchain: ${{ matrix.rust }}
-          override: true
+      - run: rustup show
       - run: rustup component add clippy
       - uses: actions-rs/cargo@v1
         with: