Use GCC 13 in the CI bots to avoid GCC 12 bug.

On GCC 12, a mutable variable can't be used in a constexpr context, but
it now works in GCC 13.

Unfortunately, (or fortunately?) GCC 13 reports an OOB write in fmtlib's
float formatting. We disable that warning for now so that we can build,
and it is reported upstream at fmtlib/fmt#3533

.github/workflows/ci.yml

@@ -51,8 +51,8 @@ jobs:
               artifact: "Linux-GCC.tar.xz",
               os: ubuntu-latest,
               build_type: "Release",
-              cc: "gcc-12",
-              cxx: "g++-12",
+              cc: "gcc-13",
+              cxx: "g++-13",
             }
 
           # Clang doesn't have good enough C++20 support to compile this library
@@ -76,6 +76,15 @@ jobs:
           chmod +x llvm.sh
           sudo ./llvm.sh 16 all
 
+      - name: Install GCC 13
+        if: matrix.config.name == 'Ubuntu GCC'
+        run: |
+          sudo add-apt-repository ppa:ubuntu-toolchain-r/ppa
+          sudo apt update
+          sudo apt-get install g++-13
+          # GCC 13 and fmtlib error: https://github.com/fmtlib/fmt/issues/3533
+          echo "CXXFLAGS=$CXXFLAGS -Wno-stringop-overflow" >> $GITHUB_ENV
+
       - name: Install Python3
         if: startsWith(matrix.config.os, 'ubuntu-')
         run: |

.github/workflows/try.yml

@@ -52,8 +52,8 @@ jobs:
               name: "Ubuntu GCC",
               os: ubuntu-latest,
               build_type: "Release",
-              cc: "gcc-12",
-              cxx: "g++-12",
+              cc: "gcc-13",
+              cxx: "g++-13",
             }
 
           # Clang doesn't have good enough C++20 support to compile this library
@@ -77,6 +77,15 @@ jobs:
           chmod +x llvm.sh
           sudo ./llvm.sh 16 all
 
+      - name: Install GCC 13
+        if: matrix.config.name == 'Ubuntu GCC'
+        run: |
+          sudo add-apt-repository ppa:ubuntu-toolchain-r/ppa
+          sudo apt update
+          sudo apt-get install g++-13
+          # GCC 13 and fmtlib error: https://github.com/fmtlib/fmt/issues/3533
+          echo "CXXFLAGS=$CXXFLAGS -Wno-stringop-overflow" >> $GITHUB_ENV
+  
       - name: Install Python3
         if: startsWith(matrix.config.os, 'ubuntu-')
         run: |