ci: Actually use toolchain in nightly/MSRV runs

Somehow at some point, additional rust toolchains became available in
CI. Thus, it's no longer sufficient to install the matrix toolchain, but
actually override it for the build directory.

Author: Naseschwarz

.github/workflows/ci.yml

@@ -41,6 +41,9 @@ jobs:
         toolchain: ${{ matrix.rust }}
         components: clippy
 
+    - name: Override rust toolchain
+      run: rustup override set ${{ matrix.rust }}
+
     - name: Rustup Show
       run: rustup show
 
@@ -112,6 +115,12 @@ jobs:
     - name: Manually install target
       run: rustup target add x86_64-unknown-linux-musl
 
+    - name: Override rust toolchain
+      run: rustup override set ${{ matrix.rust }}
+
+    - name: Rustup Show
+      run: rustup show
+
     - name: Setup MUSL
       run: |
         sudo apt-get -qq install musl-tools
@@ -166,6 +175,12 @@ jobs:
         echo "$GITHUB_WORKSPACE/gcc-arm-8.2-2018.08-x86_64-aarch64-linux-gnu/bin" >> $GITHUB_PATH
         echo "$GITHUB_WORKSPACE/gcc-arm-8.2-2018.08-x86_64-arm-linux-gnueabihf/bin" >> $GITHUB_PATH
 
+    - name: Override rust toolchain
+      run: rustup override set ${{ matrix.rust }}
+
+    - name: Rustup Show
+      run: rustup show
+
     - name: Build Debug
       run: |
         make build-linux-arm-debug
@@ -195,9 +210,16 @@ jobs:
       uses: dtolnay/rust-toolchain@master
       with:
         toolchain: ${{ matrix.rust }}
+
     - name: Setup target
       run: rustup target add x86_64-apple-darwin
 
+    - name: Override rust toolchain
+      run: rustup override set ${{ matrix.rust }}
+
+    - name: Rustup Show
+      run: rustup show
+
     - name: Build Debug
       run: |
         make build-apple-x86-debug