[ci] Better filtering for discord notifications

[poteto]

We don't always consistently use "tags" in commit messages, so let's use the filepaths modified in the PR instead to determine which channel to send notifications to.

[react-sizebot]

Comparing: 5b51a2b...2aaa253

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	+0.05%	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	514.24 kB	514.24 kB	=	91.74 kB	91.74 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	=	556.18 kB	556.18 kB	=	98.72 kB	98.72 kB
facebook-www/ReactDOM-prod.classic.js	=	595.79 kB	595.79 kB	=	104.85 kB	104.85 kB
facebook-www/ReactDOM-prod.modern.js	=	586.21 kB	586.21 kB	=	103.30 kB	103.30 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 447131a