build(deb): add s390x deb variant + reuse target dir on native runner

- Add [package.metadata.deb.variants.s390x-unknown-linux-gnu] so cargo-deb
  --variant s390x-unknown-linux-gnu succeeds (was missing; deb packaging failed).
- s390x-build-verify: set checkout clean=false to preserve the cargo target dir
  between runs on the persistent runner, reusing the fat-LTO release build.

Author: Binh Nguyen

.github/workflows/s390x-build-verify.yml

@@ -37,6 +37,10 @@ jobs:
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           ref: ${{ inputs.git_ref }}
+          # Preserve the cargo target/ dir between runs on this persistent runner
+          # so the slow fat-LTO release build is reused (cargo refingerprints on
+          # source changes). Avoids a ~1h full rebuild every packaging iteration.
+          clean: false
       - name: Ensure swap headroom (avoid OOM during compile/LTO link)
         run: |
           # Idempotent on the persistent self-hosted runner: only create the CI

Cargo.toml

@@ -95,6 +95,9 @@ depends = "libc6 (>= 2.18)"
 [package.metadata.deb.variants.aarch64-unknown-linux-musl]
 depends = ""
 
+[package.metadata.deb.variants.s390x-unknown-linux-gnu]
+depends = "libc6 (>= 2.17)"
+
 [workspace]
 members = [
   ".",