CI: Replace deprecated/unmaintained actions-rs with simple run: steps

The `actions-rs` containers haven't been maintained and updated for
years and don't need to: GitHub's actions environment already comes
fully loaded with a complete stable Rust installation with the standard
tools (clippy, rustfmt, rustdoc).  Simple `run:` commands relate
directly to what a developer can type in locally to "reproduce" the
CI environment while they might be following up on CI failures, and no
longer spam ancient Node 12 deprecation warnings.

Author: MarijnS95

.github/workflows/ci.yaml

@@ -8,48 +8,28 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
-        rust: [stable]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2
-      - uses: actions-rs/toolchain@v1
-        with:
-          toolchain: ${{ matrix.rust }}
-          override: true
       - name: Cargo build
-        uses: actions-rs/cargo@v1
-        with:
-          command: build
-          args: --workspace --all-targets
+        run: cargo build --workspace --all-targets
   test:
     name: Test
     strategy:
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
-        rust: [stable]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2
-      - uses: actions-rs/toolchain@v1
-        with:
-          toolchain: ${{ matrix.rust }}
-          override: true
       - name: Cargo test
-        uses: actions-rs/cargo@v1
-        with:
-          command: test
-          args: --workspace --all-targets
+        run: cargo test --workspace --all-targets
   lint:
     name: Lint
     strategy:
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
-        rust: [stable]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2
       - name: Cargo clippy
-        uses: actions-rs/cargo@v1
-        with:
-          command: clippy
-          args: --workspace --all-targets -- -D warnings
+        run: cargo clippy --workspace --all-targets -- -D warnings