The correct option is Emit workflow commands with the :: prefix from the step's run script to change the runner environment for that step only.

This approach instructs the runner while the step executes so the changes take effect only inside that step's shell. Each step runs in its own process which means any variables you set or PATH adjustments you make within that step remain local to it and later steps are left unchanged. This satisfies the need to customize the environment for a single step without touching the rest of the job.

Be aware that some older commands using the same prefix were deprecated for security reasons. Current guidance favors environment files for cross step persistence which would not be appropriate when you want isolation, so keeping changes inside the step process is the safe choice for this scenario.

Set the environment key at the job level in the workflow file is incorrect because a job level environment applies to every step in the job and would modify other steps.

Cloud Build is incorrect because it is a different service and does not configure a GitHub Actions step on a hosted runner.

Insert an additional setup step that exports variables for the job is incorrect because adding another step changes the job when the requirement is to leave other steps untouched, and exports from one step do not persist to later steps unless you use environment files which would then affect other steps.

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