.github/workflows/gh-pages.yml: cache mdbook build

.github/workflows/gh-pages.yml

@@ -2,14 +2,26 @@ name: github pages
 
 on:
   push:
-    branches:
-      - master
+    tags:
+      - '*'
 
 jobs:
   deploy:
     runs-on: ubuntu-18.04
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
+
+      - name: Cache cargo registry
+        uses: actions/cache@v1
+        with:
+          path: ~/.cargo/registry
+          key: ${{ runner.os }}-cargo-registry-${{ hashFiles('**/Cargo.lock') }}
+
+      - name: Cache cargo index
+        uses: actions/cache@v1
+        with:
+          path: ~/.cargo/git
+          key: ${{ runner.os }}-cargo-index-${{ hashFiles('**/Cargo.lock') }}
 
       - name: Build mdbook
         run: cargo install mdbook