[ghactions] For pull_request events use target branch name in the triqs clone command

Author: Wentzell

.github/workflows/build.yml

@@ -103,8 +103,9 @@ jobs:
       env:
         CC: ${{ matrix.cc }}
         CXX: ${{ matrix.cxx }}
+        TRIQS_BRANCH: ${{ github.event_name == 'pull_request' && github.base_ref || github.ref_name }}
       run: |
-        git clone https://github.com/TRIQS/triqs --branch unstable
+        git clone https://github.com/TRIQS/triqs --branch $TRIQS_BRANCH
         mkdir triqs/build && cd triqs/build
         cmake .. -DBuild_Tests=OFF -DCMAKE_INSTALL_PREFIX=$HOME/install
         make -j1 install VERBOSE=1