chore(pipeline): Use SSH for perf-test git submodule

[adambollen]

Description

There's an auth issue when Concourse attempts to pull the driver-perf-util repo as a submodule via HTTP (no username), so I'm changing the .gitmodules file to use SSH.

Testing

I ran the following commands locally and they succeeded:

# git submodule sync
# git submodule update --recursive --remote

Change types

• • Bug fix (non-breaking change that fixes an issue)
• • New feature (non-breaking change that adds functionality)
• • Breaking change (backwards-incompatible fix or feature)

Checklist:

• • My code follows the code style of this project.
• • My change requires a change to Fauna documentation.
• • My change requires a change to the README, and I have updated it accordingly.

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