fix(ci): recreate venv after cache restore to fix absolute symlinks

Running python3 -m venv venv over an existing cached venv recreates
the absolute symlinks and pyvenv.cfg paths so Python resolves to
the current system's python3 binary. Without this, cp -a preserves
stale symlinks from the cache source, causing venv/bin/python to
resolve to /usr/bin/python3 (system, not venv) and pytest not found.

Also switch validation to use python3 to match Ubuntu 24.04 convention.

dashboard/frontend/coverage/lcov-report/prettify.css

@@ -0,0 +1 @@
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