Set Percy's target branch to the PR base [v4.x] #9205

Job	Run time
Install (ubuntu-latest)	13s
Install (windows-latest)	1m 1s
Build (ubuntu-latest)	22s
Build (windows-latest)	1m 21s
Lint Sass (ubuntu-latest)	23s
Lint JavaScript (ubuntu-latest)	37s
EditorConfig (ubuntu-latest)	19s
Prettier (ubuntu-latest)	20s
Lint Sass (windows-latest)	1m 5s
Lint JavaScript (windows-latest)	1m 19s
EditorConfig (windows-latest)	1m 15s
Prettier (windows-latest)	56s
Verify package build (ubuntu-latest)	21s
Verify distribution build (ubuntu-latest)	29s
Verify package build (windows-latest)	1m 22s
Verify distribution build (windows-latest)	1m 29s
Export require, Node.js 12.18	20s
Export require, Node.js 12	23s
Export import, Node.js 12	16s
Export require, Node.js 18	14s
Export import, Node.js 18	16s
Nunjucks macro tests (ubuntu-latest)	41s
JavaScript unit tests (ubuntu-latest)	33s
JavaScript behaviour tests (ubuntu-latest)	1m 0s
JavaScript component tests (ubuntu-latest)	56s
Accessibility tests (ubuntu-latest)	2m 7s
Nunjucks macro tests (windows-latest)	1m 52s
JavaScript unit tests (windows-latest)	1m 6s
JavaScript behaviour tests (windows-latest)	2m 1s
JavaScript component tests (windows-latest)	2m 4s
Accessibility tests (windows-latest)	4m 14s
Percy / Send screenshots	57s
	31m 52s

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