Allow installing extra packages in before_install.sh

This is useful now that it uses different commands for installing
packages under different systems, as it means that we don't have to
write these commands manually in the CI configuration files.

.circleci/config.yml

@@ -9,15 +9,14 @@ jobs:
       - checkout
 
       - run:
-          name: Checkout required submodules
-          # We need to get Git first as it's not part of the base image.
+          name: Install dependencies
           command: |
-            dnf -y install git
-            git submodule update --init 3rdparty/catch 3rdparty/nanosvg
+            WX_EXTRA_PACKAGES=git ./build/tools/before_install.sh
 
       - run:
-          name: Install dependencies
-          command: ./build/tools/before_install.sh
+          name: Checkout required submodules
+          command: |
+            git submodule update --init 3rdparty/catch 3rdparty/nanosvg
 
       - run:
           name: Configure