fix: pass service account directly to run step

[davidcavazos]

Description

It turns out that setup-custard overrides and clears up the SERVICE_ACCOUNT variable.
Even though the input is referenced, it's not defined and can't be passed.

This will no longer be an issue when GoogleCloudPlatform/cloud-samples-tools#49 merges, and it will handle the environment variables as expected.

Once that merges, we can get rid of the setup-custard action altogether.
Instead of going through the whole cycle of fixing that action and releasing a new version, we pass the SERVICE_ACCOUNT directly to the step running the tests as a workaround.
This can go away once we use the TS runner.

Checklist

• I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• Tests pass: npm test (see Testing)
• Lint pass: npm run lint (see Style)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new sample directory, and I created GitHub Actions workflow for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[gemini-code-assist]

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