Use yarn run in Flight fixture

[eps1lon]

Fixes a bug I'm getting when running yarn dev in Node.js 22.14

$ concurrently "npm run dev:region" "npm run dev:global"
[0] npm error Invalid property "node"
[0] npm error A complete log of this run can be found in: /Users/sebbie/.npm/_logs/2025-08-13T13_12_02_245Z-debug-0.log
[1] npm error Invalid property "node"
[1] npm error A complete log of this run can be found in: /Users/sebbie/.npm/_logs/2025-08-13T13_12_02_245Z-debug-0.log
[0] npm run dev:region exited with code 1
[1] npm run dev:global exited with code 1
error Command failed with exit code 1.

Since we're using Yarn, we should really be consistent about it.

[react-sizebot]

Comparing: 9433fe3...178909d

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.68 kB	6.68 kB	=	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	530.18 kB	530.18 kB	=	93.39 kB	93.39 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	+0.05%	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	654.55 kB	654.55 kB	=	115.11 kB	115.11 kB
facebook-www/ReactDOM-prod.classic.js	=	674.32 kB	674.32 kB	=	118.29 kB	118.29 kB
facebook-www/ReactDOM-prod.modern.js	=	664.74 kB	664.74 kB	=	116.65 kB	116.65 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 178909d

[unstubbable]

Wanna fix the other five fixtures as well that use this pattern?

[eps1lon]

🙉