ci: fix wrong toolchain usage

Since the Rust version has been pinned down the CI workflow breaks due
to cargo picking up the file and using 1.34.2 for all matrix
configurations. The solution is to use the @action-rs/toolchain actions
to force a specific toolchain for the CI job.

Author: Sven

.github/workflows/rust.yml

@@ -2,34 +2,38 @@ name: Rust CI
 
 on:
   push:
-    branches: [ master, github-actions ]
+    branches: [master, github-actions]
   pull_request:
-    branches: [ master ]
+    branches: [master]
 
 jobs:
   build:
-
     runs-on: ubuntu-latest
 
     strategy:
       matrix:
         rust: ["1.34.2", stable, beta, nightly]
         features: ["", "rayon"]
         command: [test, benchmark]
+
     steps:
-    - uses: actions/checkout@v2
-    - run: rustup default ${{ matrix.rust }}
-    - name: build
-      run: >
-        cargo build --verbose --no-default-features --features "$FEATURES"
-    - name: test
-      run: >
-        cargo test --tests --benches --no-default-features --features "$FEATURES"
-      if: ${{ matrix.command == 'test' && matrix.rust != '1.34.2' }}
-      env:
-        FEATURES: ${{ matrix.features }}
-    - name: benchmark
-      run: cargo bench --bench decoding_benchmark --no-default-features --features "$FEATURES" -- --warm-up-time 1 --measurement-time 1 --sample-size 25
-      if: ${{ matrix.command == 'benchmark' && matrix.rust != '1.34.2' }}
-      env:
-        FEATURES: ${{ matrix.features }}
+      - name: Installing Rust toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          override: true
+          toolchain: ${{ matrix.rust }}
+      - uses: actions/checkout@v2
+      - name: build
+        run: >
+          cargo build --verbose --no-default-features --features "$FEATURES"
+      - name: test
+        run: >
+          cargo test --tests --benches --no-default-features --features "$FEATURES"
+        if: ${{ matrix.command == 'test' && matrix.rust != '1.34.2' }}
+        env:
+          FEATURES: ${{ matrix.features }}
+      - name: benchmark
+        run: cargo bench --bench decoding_benchmark --no-default-features --features "$FEATURES" -- --warm-up-time 1 --measurement-time 1 --sample-size 25
+        if: ${{ matrix.command == 'benchmark' && matrix.rust != '1.34.2' }}
+        env:
+          FEATURES: ${{ matrix.features }}