Release build_web_compilers 4.1.2.

[davidmorgan]

The breaking build_test changes are not released, but it's not actually needed as it's a dev dependency.

Publish this so the wasm startup change, independent of recent refactorings, is published.

[github-actions]

PR Health

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:build	2.4.3-wip	WIP (no publish necessary)
package:build_config	1.1.3-wip	WIP (no publish necessary)
package:build_daemon	4.0.5-wip	WIP (no publish necessary)
package:build_modules	5.0.12-wip	WIP (no publish necessary)
package:build_resolvers	2.4.5-wip	WIP (no publish necessary)
package:build_runner	2.4.16-wip	WIP (no publish necessary)
package:build_runner_core	8.0.1-wip	WIP (no publish necessary)
package:build_test	3.0.0-wip	WIP (no publish necessary)
package:build_web_compilers	4.1.2	ready to publish	build_web_compilers-v4.1.2
package:scratch_space	1.0.3-wip	WIP (no publish necessary)

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