Backport of "Remove GIT_CONFIG_GLOBAL=/dev/null from Duo Workflow git hardening"

What does this MR do and why?

Backport of !241085 (merged) to 19-1-stable-ee.

Removes GIT_CONFIG_GLOBAL: '/dev/null' from the git environment variables injected into Duo Workflow workload jobs in StartWorkflowService.

Since 18.11 (!226743 (merged)), this hardening caused the runner's git config --global http.<host>.sslCAInfo write during get_sources to silently go to /dev/null. As a result, git clone failed with OpenSSL error 20 on any self-managed instance using custom or self-signed CAs. Normal CI/CD jobs were unaffected, only Duo Workflow workload pipelines hit this.

This is the minimal fix: the /dev/null global config is removed so the runner's sslCAInfo write lands in a real config file again.

Original MR: !241085 (merged)

Relates to #598725 (closed)

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