[ci] Only run sizebot on pull requests

[poteto]

Stack from ghstack (oldest at bottom):

• [ci] Update sizebot node_modules caching strategy #30393
• Add run prettier commit to .git-blame-ignore-revs #30391
• [ci] Fix sizebot job omitting build_and_lint dependency #30392
• -> [ci] Only run sizebot on pull requests #30390

Oops, forgot to port this bit over.

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
react-compiler-playground	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 18, 2024 9:26pm

[react-sizebot]

Comparing: fd2b3e1...5da1ead

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	=	1.82 kB	1.82 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	499.44 kB	499.44 kB	=	89.59 kB	89.59 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	=	504.26 kB	504.26 kB	=	90.31 kB	90.30 kB
facebook-www/ReactDOM-prod.classic.js	=	599.20 kB	599.20 kB	=	105.79 kB	105.79 kB
facebook-www/ReactDOM-prod.modern.js	=	573.37 kB	573.37 kB	=	101.68 kB	101.68 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 35e5911