ci: Chromium install fails on the ubuntu-26.04 runner #46

Problem

CI fails before any test runs. The step "Install Chromium for the render smoke tests" exits with:

Error: ERROR: Playwright does not support chromium on ubuntu26.04-x64

ubuntu-latest now resolves to the ubuntu-26.04 image (20261101.186.1), and playwright-core 1.56.1 has no Chromium build for it. The last green run (#43) was on the previous image. First seen on #45: https://github.com/umiyosh/motion-render/actions/runs/42071834939

Options

• Pin the job to runs-on: ubuntu-24.04. One line, and the Chromium version stays the same, so rendering and build: two builds of the same html film differ slightly (PSNR about 60 dB) #29 are unaffected.
• Upgrade playwright-core to a version that supports Ubuntu 26.04. This changes the bundled Chromium for every user, so the stills and frame comparisons need re-checking.

Pinning first, and upgrading separately, keeps the two apart.