windows/msi: fix msi build issue with WiX 3.7/3.8
windows/msi: fix msi build issue with WiX 3.7/3.8

The `heat` tool that gathers NPM source files wasn't getting called.
Closes #4896

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node