ci(pr-test): run fred-ssr with npx

[caugner]

Description

Fixes the pr-test workflow, running fred-ssr with npx instead of npm.

Motivation

fred-ssr is a binary provided by fred, not a script, so it cannot be run with npm.

Additional details

Related issues and pull requests

Follow-up of:

• chore(deps)!: migrate from Yarn to npm #165

Same as:

• ci(pr-test): run fred-ssr with npx content#42186
• ci(pr-test): run fred-ssr with npx translated-content#30894