[WASM] _BeforeWasmBuildApp requires EMSDK_PATH to be configured

[pranavkm]

Looks like 6.0-preview3 changes haven't been ported to main:

6.0-preview3: https://github.com/dotnet/runtime/blob/release/6.0-preview3/src/mono/wasm/build/WasmApp.targets#L196-L199
main: https://github.com/dotnet/runtime/blob/main/src/mono/wasm/build/WasmApp.targets#L201-L204

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Issue Details

---

Looks like 6.0-preview3 changes haven't been ported to main:

6.0-preview3: https://github.com/dotnet/runtime/blob/release/6.0-preview3/src/mono/wasm/build/WasmApp.targets#L196-L199
main: https://github.com/dotnet/runtime/blob/main/src/mono/wasm/build/WasmApp.targets#L201-L204

Author:	pranavkm
Assignees:	-
Labels:	area-Infrastructure-mono, untriaged
Milestone:	-

[pranavkm]

FYI @lewing \ @radical