ci: add rust-cache and fix branch name for Gitea

- Add Swatinem/rust-cache@v2 to speed up CI builds
- Change branch trigger from 'main' to 'master' to match Gitea repo

.github/workflows/ci.yml

@@ -2,7 +2,7 @@ name: CI
 
 on:
   push:
-    branches: [main]
+    branches: [master]
   pull_request:
 
 env:
@@ -13,6 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+      - uses: Swatinem/rust-cache@v2
       - uses: dtolnay/rust-toolchain@stable
         with:
           components: rustfmt, clippy