CI: Fix conan install

Author: joergbrech

.github/workflows/main.yml

@@ -23,7 +23,7 @@ jobs:
         run: |
           sudo apt-get update -qq
           sudo apt-get install -y cmake cmake-data gfortran libcurl4-openssl-dev libxslt1-dev python3-setuptools
-          sudo python3 -m pip install conan
+          sudo pip3 install conan
       - name: Build TiXI
         run: |
           mkdir build