run_test_suite.yml: Start covering GCC 15

Author: hartwork

.github/workflows/run_test_suite.yml

@@ -47,9 +47,21 @@ jobs:
           - runs-on: ubuntu-24.04
             gcc: 14
             install:
+          - runs-on: ubuntu-24.04
+            gcc: 15
+            install: binutils gcc-15
     steps:
       - uses: actions/checkout@v4
 
+      - name: Add repository "ubuntu-toolchain-r" for GCC 15
+        if: "${{ matrix.gcc == '15' }}"
+        run: |
+          set -x
+          # The repository is at home at https://launchpad.net/~ubuntu-toolchain-r/+archive/ubuntu/test .
+          # NOTE: plucky is 25.04 (not 24.04 LTS)
+          wget -O - 'https://keyserver.ubuntu.com/pks/lookup?op=get&search=0xc8ec952e2a0e1fbdc5090f6a2c277a0a352154e5' | sudo apt-key add -
+          sudo add-apt-repository 'deb https://ppa.launchpadcontent.net/ubuntu-toolchain-r/test/ubuntu plucky main'
+
       - name: Install dependencies
         run: |-
           ubuntu_packages=(