[ci] Fix permissions for direct sync branch PRs workflow

[poteto]

Because we sync built artifacts into Meta, we can't support edits from inside www/fbsource to be synced back into OSS as it would cause merge conflicts for future OSS PRs.

We have a workflow that should automatically catch and close these PRs, but it looks like this one was missing one permission.

[react-sizebot]

Comparing: 03fda05...d4b89f2

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	+0.11%	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	=	655.81 kB	655.81 kB	=	115.30 kB	115.30 kB
facebook-www/ReactDOM-prod.classic.js	=	675.58 kB	675.58 kB	=	118.54 kB	118.54 kB
facebook-www/ReactDOM-prod.modern.js	=	666.00 kB	666.00 kB	=	116.86 kB	116.87 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against d4b89f2