Fix Dart2JS adding extraneous sourcemaps to its archive when both wasm and js are enabled.

[Markzipan]

I missed adding an onlyCompiler check to the exclusion logic 🫥

See: #4230

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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	5.0.18	already published at pub.dev
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	4.3.2	ready to publish	build_web_compilers-v4.3.2
package:scratch_space	1.1.1	already published at pub.dev

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