Initialize random string per workspace factory instance

What does this MR do and why?

Prior this change "random string" was defined when the factory was loaded and not instantiated. This made the string non-random for all factories.

This commit defines this string on instance level via a transient attributes. This makes this string random per factory instantiation.

Spotted while reviewing/testing #409930.

How to set up and validate locally

In Rails console:

Before

The name suffix is static:

FactoryBot.build_stubbed(:workspace).name # => "workspace-1029-1018-mz4t9g"
FactoryBot.build_stubbed(:workspace).name # => "workspace-1044-1033-mz4t9g"

After

The name suffix is dynamic:

FactoryBot.build_stubbed(:workspace).name # => "workspace-1043-1032-pp6lvu"
FactoryBot.build_stubbed(:workspace).name # => "workspace-1058-1047-lqwvbp"

MR acceptance checklist

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