fix(efs): reference imported subnet (under feature flag)

[phuhung273]

Issue # (if applicable)

Closes #33876

Description of changes

LogicalId generation cover unresolved token case.

Hi CDK team, this is the first time I submit a PR with feature flag, here are some points:

• The flag negates another flag at the exact same position, I don't know if this is a good way or not
• The flag name sounds confusing, but I cannot think of anything else

Please let me know your advice.

Description of how you validated changes

Unit + Integ

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.98%. Comparing base (242091a) to head (3290166).
Report is 16 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #34041   +/-   ##
=======================================
  Coverage   83.98%   83.98%           
=======================================
  Files         120      120           
  Lines        6976     6976           
  Branches     1178     1178           
=======================================
  Hits         5859     5859           
  Misses       1005     1005           
  Partials      112      112           

Flag	Coverage Δ
suite.unit	83.98% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
packages/aws-cdk	∅ <ø> (∅)
packages/aws-cdk-lib/core	83.98% <ø> (ø)

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[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 3290166
• Result: SUCCEEDED
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