Minor polishing of Buildkite pipelines

.buildkite/nightly.yml

@@ -4,6 +4,7 @@ env:
 
   # Per-host variables - shared across containers on host
   macos: "aarch64-darwin"
+  linux: "x86_64-linux"
 
 steps:
   - label: 'Check auto-generated Nix on macOS'
@@ -26,39 +27,39 @@ steps:
     command: "./.buildkite/bench-restore.sh mainnet"
     timeout_in_minutes: 1200
     agents:
-      system: x86_64-linux
+      system: ${linux}
       queue: adrestia-bench
     if: 'build.env("step") == null || build.env("step") =~ /restore-mainnet/'
 
   - label: 'API benchmark'
     command: "./.buildkite/bench-api.sh"
     timeout_in_minutes: 210
     agents:
-      system: x86_64-linux
+      system: ${linux}
       queue: adrestia-bench
     if: 'build.env("step") == null || build.env("step") =~ /bench-api/'
 
   - label: 'Database benchmark'
     command: "./.buildkite/bench-db.sh"
     timeout_in_minutes: 240
     agents:
-      system: x86_64-linux
+      system: ${linux}
       queue: adrestia-bench
     if: 'build.env("step") == null || build.env("step") =~ /bench-db/'
 
   - label: 'Latency benchmark'
     command: "./.buildkite/bench-latency.sh"
     timeout_in_minutes: 120
     agents:
-      system: x86_64-linux
+      system: ${linux}
       queue: adrestia-bench
     if: 'build.env("step") == null || build.env("step") =~ /bench-latency/'
 
   - label: 'Memory benchmark'
     command: "./.buildkite/bench-memory.sh"
     timeout_in_minutes: 30
     agents:
-      system: x86_64-linux
+      system: ${linux}
       queue: adrestia-bench
     if: 'build.env("step") == null || build.env("step") =~ /bench-memory/'
 

.buildkite/pipeline.yml

@@ -191,8 +191,8 @@ steps:
       system: ${macos}
 
   - label: 'Run unit tests (macOS, arm64)'
+    key: macos-tests-arm64
     depends_on: macos-nix
-    key: macos-build-tests-arm64
     command: 'nix build -L .#ci.aarch64-darwin.tests.run.unit'
     agents:
       system: ${macos}