fix file_system_locations dynamic block issue

[dkuznetsov01]

what

• Fixed the file_system_locations dynamic block to properly iterate over the list of maps
• Changed for_each from [""] to var.file_system_locations to correctly handle EFS mount configurations
• Removed unnecessary lookup() calls and used direct value access instead

why

• The current implementation causes Terraform errors when file_system_locations is provided because for_each = [""] creates an iteration over a string instead of the actual configuration maps
• This prevents users from attaching EFS file systems to CodeBuild projects, breaking a core feature of the module
• The fix allows proper iteration over each file system location map, enabling EFS mounts to work as intended

references

• Closes module file_system_locations dynamic block issue #152