Re-adding multiRootScheme as an export

[Markzipan]

This was mistakenly removed in #4240

Causing issues, such as in #4271

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:build	4.0.2	already published at pub.dev
package:build_config	1.2.0	already published at pub.dev
package:build_daemon	4.1.0	already published at pub.dev
package:build_modules	5.1.1	ready to publish	build_modules-v5.1.1
package:build_runner	2.10.2-wip	WIP (no publish necessary)
package:build_test	3.5.2-wip	WIP (no publish necessary)
package:build_web_compilers	4.4.1	ready to publish	build_web_compilers-v4.4.1
package:scratch_space	1.2.0	already published at pub.dev

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[github-actions]

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