CI: use `-funsigned-char` in one GCC build (issue #1715)

.github/workflows/ci.yml

@@ -40,6 +40,7 @@ jobs:
           - gcc: "9"
             cxxflags: -fsanitize=address -fno-sanitize-recover=all
           - gcc: "10"
+            cxxflags: -funsigned-char # Issue #1715
           - gcc: "11"
     steps:
       - name: Install
@@ -47,10 +48,10 @@ jobs:
           sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ trusty main'
           sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ trusty universe'
           sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ xenial main'
-          sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ xenial universe' 
+          sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ xenial universe'
           sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ bionic main'
-          sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ bionic universe' 
-          sudo add-apt-repository -yn 'deb http://mirrors.kernel.org/ubuntu hirsute main universe' 
+          sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ bionic universe'
+          sudo add-apt-repository -yn 'deb http://mirrors.kernel.org/ubuntu hirsute main universe'
           sudo apt-get update
           sudo apt-get install -y gcc-${{ matrix.gcc }} g++-${{ matrix.gcc }}
       - name: Checkout
@@ -103,9 +104,9 @@ jobs:
           sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ trusty main'
           sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ trusty universe'
           sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ xenial main'
-          sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ xenial universe' 
+          sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ xenial universe'
           sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ bionic main'
-          sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ bionic universe' 
+          sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ bionic universe'
           sudo apt-get update
           sudo apt-get install -y clang-${{ matrix.clang }}
       - name: Checkout