C/C++ CI: use GCC 11 for the ubuntu-latest build

.github/workflows/c-cpp.yml

@@ -9,15 +9,20 @@ on:
 jobs:
   build-ubuntu:
 
-    runs-on: ${{ matrix.os }}
+    runs-on: ${{ matrix.config.os }}
 
     strategy:
       matrix:
-        os: [ubuntu-latest, ubuntu-18.04]
+        config:
+          - {os: ubuntu-latest, c: gcc-11, cpp: g++-11}
+          - {os: ubuntu-18.04, c: gcc, cpp: g++}
 
     steps:
     - name: Install dependencies
-      run: sudo apt update && sudo apt install git build-essential cmake libuv1-dev libzmq3-dev libsodium-dev libpgm-dev libnorm-dev libgss-dev
+      run: |
+        sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
+        sudo apt update
+        sudo apt install -y git build-essential cmake libuv1-dev libzmq3-dev libsodium-dev libpgm-dev libnorm-dev libgss-dev ${{ matrix.config.c }} ${{ matrix.config.cpp }}
 
     - name: Checkout repository
       uses: actions/checkout@v2
@@ -28,7 +33,7 @@ jobs:
       run: |
         mkdir build
         cd build
-        cmake ..
+        cmake .. -DCMAKE_C_COMPILER=${{ matrix.config.c }} -DCMAKE_CXX_COMPILER=${{ matrix.config.cpp }}
         make -j2
 
     - name: Build tests
@@ -36,7 +41,7 @@ jobs:
         cd tests
         mkdir build
         cd build
-        cmake ..
+        cmake .. -DCMAKE_C_COMPILER=${{ matrix.config.c }} -DCMAKE_CXX_COMPILER=${{ matrix.config.cpp }}
         make -j2
 
     - name: Run tests
@@ -45,7 +50,7 @@ jobs:
     - name: Archive binary
       uses: actions/upload-artifact@v2
       with:
-        name: p2pool-${{ matrix.os }}
+        name: p2pool-${{ matrix.config.os }}
         path: build/p2pool
 
   build-windows-msys2: