fix: Ensure that coverage file generation is atomic. (#5426)

Running unit tests in parallel and multiple threads can write into one file can corrupt output files, and then gcovr won't be able to parse the corrupted file. This change adds -fprofile-update=atomic as instructed by https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68080.

.github/workflows/nix.yml

@@ -247,7 +247,7 @@ jobs:
           mkdir -p ~/.conan
           tar -xzf conan.tar -C ~/.conan
       - name: install gcovr
-        run: pip install "gcovr>=7,<8"
+        run: pip install "gcovr>=7,<9"
       - name: check environment
         run: |
           echo ${PATH} | tr ':' '\n'