[ci] Set disk size for artifact builds (elastic#197663)

Defines disk size for artifact builds. This will be a no-op - the boot
disk size is >= the definitions in this PR.

A test run with the smaller boot disk can be seen in
https://buildkite.com/elastic/kibana-pull-request/builds/248242. I plan
on making further adjustments after the boot disk has been promoted.

(cherry picked from commit 316efc7)

.buildkite/pipelines/artifacts.yml

@@ -6,6 +6,7 @@ steps:
       imageProject: elastic-images-prod
       provider: gcp
       machineType: c2-standard-16
+      diskSizeGb: 125
     timeout_in_minutes: 120
     retry:
       automatic:

.buildkite/pipelines/pull_request/base.yml

@@ -12,6 +12,7 @@ steps:
     agents:
       machineType: n2-standard-16
       preemptible: true
+      diskSizeGb: 125
     key: build
     if: "build.env('KIBANA_BUILD_ID') == null || build.env('KIBANA_BUILD_ID') == ''"
     timeout_in_minutes: 90