Prevent running validate when pushing on main

Rational is as follow:
    - We ran validate-book on each push on the PR branch anyway
    - We will run release_book which is a superset of validate

.github/workflows/validate-book-build.yml

@@ -1,5 +1,9 @@
 name: Validate LaTeX Document
 on: [ push, pull_request ]
+  branches:
+    - '**'  # All branches
+    - '!main' # But main as will do it anyway and more in release_book
+
 jobs:
   test_building_book:
     runs-on: ubuntu-latest