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**What I did**
Remove usage of `pip3` from the vs test pipeline script following the removal of `python3-pip` from the CI agent. Replace all `pip3 install` calls with `uv pip install --system` to maintain equivalent system-wide package installation behaviour.

**Why I did it**
`python3-pip` was removed from CI agent because S360 flags python3-pip as a security vulnerability

**How I verified it**

**Details if related**

Signed-off-by: Sonic Build Admin <sonicbld@microsoft.com>