[build_web_compilers] Fix WebAssembly detection logic

[kevmoo]

Fixes #3930

[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	already published at pub.dev
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	9.0.0-wip	WIP (no publish necessary)
package:build_test	3.0.0-wip	WIP (no publish necessary)
package:build_web_compilers	4.1.3	ready to publish	build_web_compilers-v4.1.3
package:scratch_space	1.0.3-wip	WIP (no publish necessary)

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

[github-actions]

PR Health

Changelog Entry ✔️

Package	Changed Files

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

[davidmorgan]

Thanks both :)