update the workflow

Author: mili1247

.github/workflows/build.yml

@@ -81,7 +81,7 @@ jobs:
       run: |
         git clone https://github.com/TRIQS/triqs --branch unstable
         mkdir triqs/build && cd triqs/build
-        cmake .. -DBuild_Tests=OFF -DCMAKE_INSTALL_PREFIX=$HOME/install
+        cmake .. -DBuild_Tests=OFF -DCMAKE_INSTALL_PREFIX=$HOME/install -DPYTHON_EXECUTABLE=/usr/local/bin/python3
         make -j1 install VERBOSE=1
         cd ../