Fix CMake workflow for PRs.

We need to use the PR repo, not the upstream in the codelab test.

Context: https://github.com/google/fuzztest/pull/1711#issuecomment-2903719057
PiperOrigin-RevId: 763113538

.github/workflows/cmake_test.yml

diff --git a/.github/workflows/cmake_test.yml b/.github/workflows/cmake_test.yml
index d23c692..f9da144 100644
--- a/.github/workflows/cmake_test.yml
+++ b/.github/workflows/cmake_test.yml
@@ -63,7 +63,7 @@
             -D CMAKE_C_COMPILER_LAUNCHER=ccache \
             -D CMAKE_CXX_COMPILER_LAUNCHER=ccache \
             -D CMAKE_BUILD_TYPE=RelWithDebug \
-            -D FUZZTEST_REPO="${{ github.repository }}" \
+            -D FUZZTEST_REPO="${{ github.event.pull_request.head.repo.full_name }}" \
             -D FUZZTEST_REPO_BRANCH="${{ github.head_ref || github.ref_name }}" \
           && cmake --build build_codelab -j $(nproc)
       - name: Run all tests in default mode with clang