Addresses #742, environment variable setup issue

[insop]

Using a default value for USER prevents failures in environments where it’s unset (e.g., many containers). This change worked for me, and I can open a PR to apply it to other config files if desired.

• I have personally reviewed this PR and description before asking others to do so. It meets the quality bar I expect from others. I understand that if this PR is perceived as unverified AI-generated code, it will be closed without further explanation.
• I have run tests and confirmed that this code works. I ran this code for apps/grpo

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Description

Addresses #742

Using a default value for USER prevents failures in environments where it’s unset (e.g., many containers). This change worked for me, and I can open a PR to apply it to other config files if desired.

Test plan

• I ran apps/grpo with this code.
• pre-commit run --all-files ran clean

[insop]

@daniellepintz , PTAL.