feat: add per-recipe shell script support (#38)

build.sh

@@ -1,4 +1,13 @@
 #!/bin/bash
+# run scripts
+echo "-- Running scripts defined in recipe.yml --"
+buildscripts=$(yq '.scripts[]' < /tmp/ublue-recipe.yml)
+for script in $(echo -e "$buildscripts"); do \
+    echo "Running: ${script}" && \
+    /tmp/scripts/$script; \
+done
+echo "---"
+
 # remove the default firefox (from fedora) in favor of the flatpak
 rpm-ostree override remove firefox firefox-langpacks