Update Swatinem/rust-cache action to v2.7.2

Author: renovate[bot]

.github/workflows/main.yml

@@ -11,7 +11,7 @@ jobs:
     steps:
       - uses: actions/checkout@v2
 
-      - uses: Swatinem/rust-cache@v2.5.0
+      - uses: Swatinem/rust-cache@a22603398250b864f7190077025cf752307154dc # v2.7.2
 
       - run: cargo run
       - run: cp CNAME ./site/