Refactor environment variable handling in gitlab-tofu

See merge request components/opentofu!189

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Please use `TF_HTTP_USERNAME` and `TF_HTTP_PASSWORD` environment
variables directly.

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Simplify dummy jobs for delete-state job

See merge request components/opentofu!194

Use `include:project` instead of local for child pipeline triggers

See merge request components/opentofu!193

Add more integration tests for trigger in child pipeline

See merge request components/opentofu!191

Support running pipeline templates directly in child pipeline with `trigger_in_child_pipeline` input

See merge request components/opentofu!190

Support running pipeline templates directly in child pipeline with `trigger_in_child_pipeline` input

This change set allows a user of this component to directly execute a
pipeline template in its own child pipeline. This is useful to run
dedicated child pipeline for multiple environments.

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Rename `TF_GITLAB_SOURCED` variable to `GITLAB_TOFU_SOURCED`

See merge request components/opentofu!188

This change set renames the `TF_GITLAB_SOURCED` environment variable to
`GITLAB_TOFU_SOURCED` and document the existence of the variable
accordingly in the README.
The reason for renaming it is that the `TF_` variable prefix is "owned"
by OpenTofu / Terraform and not GitLab Tofu. Therefore, we make an
effort to rename these variables to have the `GITLAB_TOFU_` prefix
instead.

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