ci: add the OTEL_SDK_DISABLED fleet-env knob

FLEET_ENV gained the knob and telemetry-env-is-disabled asserts every
one at CI runtime, but ci.yml is a seed-once preset the cascade never
updates, so each repo needs it added directly.

Author: jdalton

.github/workflows/ci.yml

@@ -36,6 +36,7 @@ env:
   # fleet-wide burn-down took every member to zero findings.
   LINT_MARKDOWN: '1'
   NO_UPDATE_NOTIFIER: '1'
+  OTEL_SDK_DISABLED: 'true'
 
 jobs:
   # First step of every job is an inline git-fetch bootstrap (no third-party