Re-enable MSan build in GitHub Actions CI

Replace the old commented-out MSan configuration (ubuntu-18.04 with
clang-6.0) with a modern configuration using ubuntu-latest and clang-18,
matching the other sanitizer builds (asan, lsan, tsan, ubsan).

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

Author: etr

.github/workflows/verify-build.yml

@@ -62,17 +62,15 @@ jobs:
             cc-compiler: clang++-18
             debug: debug
             coverage: nocoverage
-          # This test gives false positives on newer versions of clang
-          # and ubuntu-18.04 is not supported anymore on github
-          #- test-group: extra
-          #  os: ubuntu-18.04
-          #  os-type: ubuntu
-          #  build-type: msan
-          #  compiler-family: clang
-          #  c-compiler: clang-6.0
-          #  cc-compiler: clang++-6.0
-          #  debug: debug
-          #  coverage: nocoverage
+          - test-group: extra
+            os: ubuntu-latest
+            os-type: ubuntu
+            build-type: msan
+            compiler-family: clang
+            c-compiler: clang-18
+            cc-compiler: clang++-18
+            debug: debug
+            coverage: nocoverage
           - test-group: extra
             os: ubuntu-latest
             os-type: ubuntu