[ci] Disallow PRs against builds branch

[poteto]

Our internal build infra relies on a 1:1 mapping between main and the 2 build branches. Directly committing changes to those branches breaks that infra.

Adds a simple workflow to leave a comment and decline the PR.

[react-sizebot]

Comparing: 569c3b2...b372815

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	=	515.48 kB	515.48 kB	=	92.04 kB	92.04 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	=	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	558.52 kB	558.52 kB	=	99.27 kB	99.27 kB
facebook-www/ReactDOM-prod.classic.js	=	636.88 kB	636.88 kB	=	111.92 kB	111.92 kB
facebook-www/ReactDOM-prod.modern.js	=	627.20 kB	627.20 kB	=	110.34 kB	110.34 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 61eede2