fix(ci): bump rust-target cache volume to unstick the plugin runtime

The wasm32-wasip2 fix (20368ff) didn't take: the Pets E2E run on
that exact commit still failed with the identical "this server was
built without the plugin runtime" 500. build.rs's build_plugin_runtime()
only declares `rerun-if-changed` on plugin-runtime/{src,wit} and the
ATOMICSERVER_SKIP_PLUGIN_RUNTIME env var — nothing tells cargo that
"the wasm32-wasip2 target just became installed" should invalidate its
build-script fingerprint. Every prior CI run had already cached an
empty embedded runtime (from before that target existed) in the
rust-target-v3 volume, so cargo kept trusting that stale fingerprint
and never re-ran the nested wasm32-wasip2 build to notice the target
was now there.

This is the same class of staleness bug TOUCH_WORKSPACE_SOURCES
already documents fixing twice before (the -v2 and -v3 renames on this
same volume, for the same "cached artifact looks newer than fresh
sources" reason) — same fix, one more rename to force a clean
re-evaluation.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KkL48idFqkuYw8n7KdH6Dj

Author: claude

.dagger/src/index.ts

@@ -1162,7 +1162,17 @@ export class AtomicServer {
       )
       .withDirectory('/code/atomic-plugin', source.directory('atomic-plugin'))
       .withDirectory('/code/tools', source.directory('tools'))
-      .withMountedCache('/code/target', dag.cacheVolume('rust-target-v3'))
+      // v3 -> v4: `atomic-server`'s build.rs only declares
+      // `rerun-if-changed` on `plugin-runtime/{src,wit}` and
+      // `ATOMICSERVER_SKIP_PLUGIN_RUNTIME` — it has no way to know "the
+      // wasm32-wasip2 target just became installed". Adding the `rustup
+      // target add` step above changed the container, but every prior CI
+      // run had already fingerprinted build.rs's output (an empty embedded
+      // runtime, from before that target existed) into this cache volume,
+      // so cargo kept trusting the stale fingerprint and never re-ran
+      // build_plugin_runtime() to notice the target was now there. Bumping
+      // the volume forces one full rebuild that actually re-evaluates it.
+      .withMountedCache('/code/target', dag.cacheVolume('rust-target-v4'))
       .withExec(TOUCH_WORKSPACE_SOURCES)
       .withWorkdir('/code')
       .withExec(['cargo', 'fetch']);