ci(host-agent): bootstrap Rust in the runner container; roll to alpha.2

Asgard runner executes jobs in bare node:20-bullseye (no Rust, no sudo)
- install rustup + musl/mingw cross toolchains per-run, same pattern as
setup-go in the Go pipeline. agent-v2.0.0-alpha.1 predates this fix;
forward-only doctrine: version rolls to alpha.2 rather than re-pushing
the tag.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

corrosion-host-agent/Cargo.lock

@@ -258,7 +258,7 @@ checksum = "773648b94d0e5d620f64f280777445740e61fe701025087ec8b57f45c791888b"
 
 [[package]]
 name = "corrosion-host-agent"
-version = "2.0.0-alpha.1"
+version = "2.0.0-alpha.2"
 dependencies = [
  "anyhow",
  "async-nats",