CI: Tweak agent spec

Author: jpsamaroo

.buildkite/pipeline.yml

@@ -2,15 +2,15 @@
   if: build.message !~ /\[skip tests\]/
   agents:
     queue: "juliaecosystem"
-    sandbox.jl: "true"
+    sandbox_capable: "true"
     os: linux
     arch: x86_64
   command: "julia --project -e 'using Pkg; Pkg.develop(;path=\"lib/TimespanLogging\")'"
 .bench: &bench
   if: build.message =~ /\[run benchmarks\]/
   agents:
     queue: "juliaecosystem"
-    sandbox.jl: "true"
+    sandbox_capable: "true"
     os: linux
     arch: x86_64
     num_cpus: 16
@@ -112,7 +112,7 @@ steps:
       JULIA_NUM_THREADS: "4"
     agents:
       queue: "juliaecosystem"
-      sandbox.jl: "true"
+      sandbox_capable: "true"
       os: linux
       arch: x86_64
     command: "git clone https://github.com/JuliaParallel/DTables.jl.git ; julia -t4 -e 'using Pkg; Pkg.activate(\"DTables.jl\"); Pkg.develop(;path=\".\"); Pkg.instantiate(); Pkg.test()'"