Pin mdbook to 0.4 in CI

There are non-backwards compatible changes in the latest 0.5 release and we
can't upgrade Acton docs because of dependencies on preprocessors (like
admonish, though it seem to have been replaced by integrated admonitision in
mdbook but also svgbob and perhaps others) that have no support for the new 0.5
format. Pinning to 0.4 is the safe choice for now. We'll have to upgrade later.

Author: Kristian Larsson

.github/workflows/main.yml

@@ -23,7 +23,7 @@ jobs:
           path: ~/.cargo
           key: cargo
       - name: 'Install mdbook'
-        run: cargo install --locked mdbook mdbook-admonish mdbook-tabs mdbook-svgbob
+        run: cargo install --locked mdbook@0.4.51 mdbook-admonish@1.20.0 mdbook-tabs@0.2.3 mdbook-svgbob@0.2.2
       - name: 'Checkout'
         uses: actions/checkout@master
         with: