github: clone imgui from github (ubuntu package not useful)

.github/workflows/main-stock-ubuntu.yml

@@ -57,10 +57,6 @@ jobs:
           sudo apt install -y sphinx
           echo "::endgroup"
 
-          echo "::group::libimgui-dev install"
-          sudo apt install -y libimgui-dev
-          echo "::endgroup"
-
           echo "::group::libvulkan-dev install"
           sudo apt install -y libvulkan-dev
           echo "::endgroup"
@@ -100,6 +96,13 @@ jobs:
 
           # this should be sufficient for find_package(Catch2) to work
 
+      - name: clone imgui
+        run: |
+
+          echo "::group::clone imgui"
+          git clone https://github.com/ocornut/imgui.git
+          echo "::endgroup"
+
       - name: list /usr/lib
         run: |
           ls /usr/lib