ci: Also build Rust nightly in Release mode

Would have caught https://materializeinc.slack.com/archives/C08ACQNGSQK/p1759754676092499

Author: def-

ci/nightly/pipeline.template.yml

@@ -72,7 +72,7 @@ steps:
         timeout_in_minutes: 60
         agents:
           queue: hetzner-aarch64-16cpu-32gb
-        branches: "main"
+        branches: "main *build*"
 
   - group: Linters
     key: linters

ci/test/rust-beta-build.sh

@@ -16,4 +16,5 @@ curl https://sh.rustup.rs -sSf | sh -s -- -y
 source /cargo/env
 trap "rustup self uninstall -y || true" SIGTERM SIGINT EXIT
 rustup install beta
+rustup run beta cargo build --all-targets --release
 rustup run beta cargo build --all-targets