[CI / FIPS] Fix env var for checks (#186565)

In #183777 review iterations `FIPS_ENABLED` was changed, but not updated
here.

`Checks` with FIPS agents enabled:
https://buildkite.com/elastic/kibana-pull-request/builds/217048#019037fd-675f-4860-839a-d5e0602a40c4/281-284

.buildkite/scripts/steps/checks.sh

@@ -5,7 +5,7 @@ set -euo pipefail
 export DISABLE_BOOTSTRAP_VALIDATION=false
 .buildkite/scripts/bootstrap.sh
 
-if [[ "${FIPS_ENABLED:-}" == "true" ]]; then
+if [[ "${FTR_ENABLE_FIPS_AGENT:-}" == "true" ]]; then
   .buildkite/scripts/steps/checks/verify_fips_enabled.sh
 fi
 .buildkite/scripts/steps/checks/saved_objects_compat_changes.sh