Use cargo install --locked

It's not possible to install mdbook-svgbob right now due to some changes
in some packages. Using --locked fixes it, presumably by using some
pinned / locked versions of dependencies!?

.github/workflows/main.yml

@@ -21,9 +21,9 @@ jobs:
         uses: actions/cache@v3
         with:
           path: ~/.cargo
-          key: cargo-zola
-      - name: 'Install mdbook & svgbob'
-        run: cargo install mdbook mdbook-svgbob
+          key: cargo
+      - name: 'Install mdbook-svgbob'
+        run: cargo install --locked mdbook-svgbob
       - name: 'Checkout'
         uses: actions/checkout@master
       - name: 'Checkout acton repo'