Build environment variables overwritten by nested manifests

[HipsterBrown]

Build environment: macOS
Moddable SDK version: 5.3..0
Target device: NodeMCU ESP8266

Description
When targeting the ESP8266, the manifest for servo support includes a build environment variable for ARDUINO_ESP8266 that is expected to be in $(USERHOME)/esp/esp8266-2.3.0/. This conflicts with the conventional setup location for xs-dev: $(USERHOME)/.local/share/esp/esp8266-2.3.0/ (or $(USERHOME)/xs-dev/esp/esp8266-2.3.0/ on Windows). Setting the ARDUINO_ESP8266 build variable in my project manifest fails to fix the issue due to the parseBuild method in mcmanifest.js setting the environment variable, whether or not it has been set before.

Steps to Reproduce

1. xs-dev setup --device esp8266
2. xs-dev run --example pins/servo --device esp8266/nodemcu
3. See error: ### Error: '$(ARDUINO_ESP8266)/libraries/Servo/src/esp8266/*': directory not found!

Expected behavior
Setting the build environment variable should resolve the module paths correctly.

Other information
Fixing this might resolve in breaking changes if other parts of the build system rely on this behavior,