Install flatpak-builder

Author: janopae

.github/workflows/deploy.yml

@@ -33,7 +33,7 @@ jobs:
       - name: Install Linux dependencies
         run: |
           sudo apt update
-          sudo apt install flatpak xsltproc docbook-xsl
+          sudo apt install flatpak flatpak-builder xsltproc docbook-xsl
       - name: Make Flatpak file
         run: |
           make dist/trelby.flatpak