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/checks.yml

@@ -28,10 +28,13 @@ jobs:
     timeout-minutes: 45
     container:
       image: ${{ needs.preflight.outputs.IMAGE }}
+      volumes:
+        - /cache/cargo-target/pezkuwi-sdk:/cache/target
     env:
       RUSTFLAGS: "-D warnings"
       SKIP_WASM_BUILD: 1
       CARGO_INCREMENTAL: 0
+      CARGO_TARGET_DIR: /cache/target
     steps:
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v4.1.7
 
@@ -56,9 +59,12 @@ jobs:
     timeout-minutes: 45
     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
     steps:
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v4.1.7