Fix plugin caches when installing Varbase Demo to ensure field_block plugins are available for optional configurations that reference them

Problem/Motivation

#3538500: Fix block plugin not found warnings during Drush installation of Varbase

Steps to reproduce

When installing Varbase with Drush

ddev install-varbase demo

Proposed resolution

• Clear plugin caches to ensure field_block plugins are available for optional configurations that reference them.

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ✅ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ✅ Code review from 1 Varbase core team member
• ✅ Full testing and approval
• ✅ Credit contributors
• ✅ Review with the product owner
• ✅ Update Release Notes and Update Helper on new feature change/addition
• ✅ Release varbase-10.1.0-beta1, varbase_demo-1.1.0-beta1

Varbase update type

• ✅ No Update
• ➖ Optional Update
• ➖ Forced Update
• ➖ Forced Update if Unchanged

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• [#3543975] fix: plugin caches when installing Varbase Demo to ensure field_block plugins are available for optional configurations that reference them