Only compile from source if needed

roles/libsodium/tasks/debian.yml

@@ -26,4 +26,4 @@
   - libsodium-dev
 
 - include: source.yml
-  when: libsodium_build_from_source
+  when: libsodium_build_from_source == 'true'

roles/libsodium/tasks/ubuntu.yml

@@ -3,7 +3,7 @@
   when: ansible_lsb.major_release|int < 15
 
 - include: source.yml
-  when: libsodium_build_from_source
+  when: libsodium_build_from_source == 'true'
 
 - name: Install libsodium via apt
   when: libsodium_build_from_source != 'true'