[wasm-mt] The `browser-wasm` runtime pack should pack both single- and multi-threaded builds.

[lambdageek]

The multithreaded build artifacts should go into an mt subfolder.

Depends on #68506

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Issue Details

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The multithreaded build artifacts should go into an mt subfolder.

Depends on #68506

Author:	lambdageek
Assignees:	-
Labels:	arch-wasm, untriaged, area-Build-mono
Milestone:	-

[lambdageek]

We ended up with separate runtime packs (due to issues with a single runtime pack manifest containing multiple files with the same name in different subdirectories)

Selectable using MonoWasmBuildVariant=perftrace or MonoWasmBuildVariant=multithread in #71054 which also publishes them