Update CI to use juliaecosystem

Author: staticfloat

.buildkite/pipeline.yml

@@ -6,3 +6,5 @@ steps:
       JULIA_VERSION=1.6     LOCAL_SANDBOX=true  buildkite-agent pipeline upload .buildkite/test_sandbox.yml
       JULIA_VERSION=nightly LOCAL_SANDBOX=false buildkite-agent pipeline upload .buildkite/test_sandbox.yml
       JULIA_VERSION=nightly LOCAL_SANDBOX=true  buildkite-agent pipeline upload .buildkite/test_sandbox.yml
+    agents:
+      queue: "juliaecosystem"

.buildkite/test_sandbox.yml

@@ -1,8 +1,3 @@
-# Only run on agents that are using sandbox.jl
-agents:
-  queue: "julia"
-  sandbox.jl: "true"
-
 steps:
   - label: ":julia: Test on Julia ${JULIA_VERSION}, local sandbox: ${LOCAL_SANDBOX}"
     plugins:
@@ -22,3 +17,7 @@ steps:
       # just disable the `binfmt_misc` tests within nested sandbox environments,
       # of which all buildkite agents are.
       SANDBOX_TEST_MULTIARCH: false
+    # Only run on agents that are already using `sandbox.jl`, as then we know we can nest
+    agents:
+      queue: "juliaecosystem"
+      sandbox.jl: "true"