Use GCC 13 for valgrind job to avoid -Woverloaded-virtual error

GCC 14 with -Werror catches a latent warning in littletest.hpp test
framework that older compilers don't flag. Use GCC 13 for now.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: etr

.github/workflows/verify-build.yml

@@ -222,8 +222,8 @@ jobs:
             os-type: ubuntu
             build-type: valgrind
             compiler-family: gcc
-            c-compiler: gcc-14
-            cc-compiler: g++-14
+            c-compiler: gcc-13
+            cc-compiler: g++-13
             debug: nodebug
             coverage: nocoverage
           - test-group: extra