maint(core,developer): add emscripten env var for building on arm64

[mcdurdin]

The version of Emscripten we are on does not currently support ARM. We can override this on Windows with:

EMSDK_ARCH=x86_64

Ref:

keyman/.github/workflows/core-arm64-windows-test.yml

Lines 94 to 95 in 22731fd

	# As of Dec 2025, Emscripten does not support Win11 ARM natively
	echo "EMSDK_ARCH=x86_64" >> $GITHUB_ENV