.github: change to ubuntu-latest, 18.04 deprecated

For details, see https://github.com/actions/runner-images/issues/6002

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

.github/workflows/build.yml

@@ -15,7 +15,7 @@ jobs:
   build:
     # Verify we can build on latest Ubuntu with both gcc and clang
     name: ${{ matrix.compiler }}
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     strategy:
       matrix:
         compiler: [gcc, clang]
@@ -27,7 +27,7 @@ jobs:
       - name: Install dependencies
         run: |
           sudo apt-get -y update
-          sudo apt-get -y install pkg-config
+          sudo apt-get -y install pkg-config tree
           wget https://github.com/troglobit/libuev/releases/download/v2.3.2/libuev-2.3.2.tar.xz
           wget https://github.com/troglobit/libite/releases/download/v2.5.2/libite-2.5.2.tar.gz
           tar xf libuev-2.3.2.tar.xz