ci: Use slightly older emscripten for now #488

Job	Run time
build (macos-latest)	28s
build_with_bundled_z3 (linux)	12m 16s
build (ubuntu-latest)	54s
build_on_wasm	25m 32s
build_with_bundled_z3 (macos)	10m 1s
build_with_vcpkg_installed_z3 (linux)	1m 33s
build_with_bundled_z3 (windows)	14m 31s
build_with_vcpkg_installed_z3 (macos)	1m 6s
check-formatting	6s
run_clippy	34s
build_with_vcpkg_installed_z3 (windows)	5m 3s
	1h 12m 4s

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