Work around frozen string Ruby 2.7 bug in slug generation

What does this MR do and why?

!117433 (merged) caused the slugified variable to be frozen in Ruby 2.7 when the "env-" prefixed were added. The following script prints true in Ruby 2.7 but false in Ruby 3.0+.

# frozen_string_literal: true

foo = "9-staging-123-q517sa"
foo = "env-#{foo}"

puts foo.frozen?

To preserve Ruby 2.7 compatibiliity, ensure the return string is unfrozen via the + operator.

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