Fix error when using array for the project keyword

What does this MR do and why?

When you use an array in the project keyword for include, like so:

include:
  - project: ["lauraX/public-project"]
    file: test.yml
    ref: master

test:
  script: echo hello

and you use a masked variable, Ci::MaskSecret throws an error because it's trying to gsub an array.

As a temporary fix, we will force the array into a string. For a permanent fix, we will validate that the project keyword use only a string. Issue: TBD

Screenshots or screen recordings

Before	After
NO PIPELINE FOR YOU	Pipeline ✅

How to set up and validate locally

1. Define a config with an include:project defined in an array:

# public-project
include:
  - project: ["lauraX/included-project"]
    file: test.yml
    ref: master

test:
  script: echo hello

2. Create a masked variable whose value is the name of the project:

3. Run a pipeline

Issue: #360624 (closed)

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