feat: add persistent cargo target cache for self-hosted runners

Mount /cache/cargo-target/pezkuwi-sdk on VPS runners to reuse
compiled artifacts between workflow runs. This significantly
speeds up subsequent builds by avoiding full recompilation.

Updated workflows:
- checks.yml (cargo-clippy, check-try-runtime)
- docs.yml (test-doc, build-rustdoc)
- tests-linux-stable.yml (all test jobs)

Also removed Swatinem/rust-cache where persistent cache is used
since it's now redundant.

.github/workflows/docs.yml

@@ -25,19 +25,16 @@ jobs:
     needs: [preflight]
     container:
       image: ${{ needs.preflight.outputs.IMAGE }}
+      volumes:
+        - /cache/cargo-target/pezkuwi-sdk:/cache/target
     env:
       RUSTFLAGS: "-Cdebug-assertions=y -Dwarnings"
       SKIP_WASM_BUILD: 1
       CARGO_INCREMENTAL: 0
+      CARGO_TARGET_DIR: /cache/target
     steps:
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
 
-      - name: Rust Cache
-        uses: Swatinem/rust-cache@9d47c6ad4b02e050fd481d890b2ea34778fd09d6 # v2.7.8
-        with:
-          shared-key: test-doc
-          cache-on-failure: true
-
       # Exclude pezkuwi-zombienet-sdk-tests: its build.rs runs nested cargo build
       # which doesn't inherit workspace [patch.crates-io] settings, causing
       # serde_core duplicate lang item error. See: https://github.com/pezkuwichain/pezkuwi-sdk/issues/357
@@ -51,19 +48,16 @@ jobs:
     needs: [preflight]
     container:
       image: ${{ needs.preflight.outputs.IMAGE }}
+      volumes:
+        - /cache/cargo-target/pezkuwi-sdk:/cache/target
     env:
       SKIP_WASM_BUILD: 1
       CARGO_INCREMENTAL: 0
+      CARGO_TARGET_DIR: /cache/target
       RUSTDOCFLAGS: "-Dwarnings --default-theme=ayu --html-in-header ./docs/sdk/assets/header.html --extend-css ./docs/sdk/assets/theme.css --html-after-content ./docs/sdk/assets/after-content.html"
     steps:
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
 
-      - name: Rust Cache
-        uses: Swatinem/rust-cache@9d47c6ad4b02e050fd481d890b2ea34778fd09d6 # v2.7.8
-        with:
-          shared-key: build-rustdoc
-          cache-on-failure: true
-
       # Exclude pezkuwi-zombienet-sdk-tests: nested cargo build in build.rs
       # doesn't inherit workspace patches. See test-doc job comment.
       - run: cargo doc --all-features --workspace --no-deps --locked --exclude pezkuwi-zombienet-sdk-tests