ci: switch to gitea.com action mirrors + rustup inline

The self-hosted act_runner cannot reach github.com (network timeout
on actions/checkout clone). Replace:

- actions/checkout@v4 -> https://gitea.com/actions/checkout@v4 (3 sites)
- actions/cache@v4    -> https://gitea.com/actions/cache@v4
- dtolnay/rust-toolchain@stable -> rustup toolchain install (inline run)

gitea.com maintains official mirrors of the actions/* org. dtolnay's
rust-toolchain is third-party (no gitea.com mirror), so replaced with
a direct rustup invocation — the act_runner ubuntu image has rustup
pre-installed.

This unblocks CI which has been red since the original PR #26 was
opened 6 weeks ago. No code changes.

.github/workflows/ci.yml

@@ -35,12 +35,10 @@ jobs:
     name: Build + Clippy + Test
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: https://gitea.com/actions/checkout@v4
 
       - name: Install Rust toolchain (stable)
-        uses: dtolnay/rust-toolchain@stable
-        with:
-          components: clippy
+        run: rustup toolchain install stable --profile minimal --component clippy && rustup default stable
 
       - name: Install system dependencies
         run: |
@@ -81,7 +79,7 @@ jobs:
           echo "LIBCLANG_PATH=$LIBDIR" >> "$GITHUB_ENV"
 
       - name: Cache cargo registry + build artifacts
-        uses: actions/cache@v4
+        uses: https://gitea.com/actions/cache@v4
         with:
           path: |
             ~/.cargo/registry
@@ -106,10 +104,10 @@ jobs:
     # Keep separate from build-test so a vulnerability advisory fails the
     # check independently of compile state.
     steps:
-      - uses: actions/checkout@v4
+      - uses: https://gitea.com/actions/checkout@v4
 
       - name: Install Rust toolchain (stable)
-        uses: dtolnay/rust-toolchain@stable
+        run: rustup toolchain install stable --profile minimal && rustup default stable
 
       - name: Install cargo-audit
         run: cargo install cargo-audit --locked