Fix buildpath regression

[cmaglie]

WIP: do not merge before arduino/arduino-cli#418

There were actually two bugs:

• the command line parsing check, to see if the build-path parameter has been passed, should be done against the empty string ("") and not against nil
• the old ctx.ToolsDirs parameter has been replaced by ctx.BuiltInToolsDirs but this has not been reflected in the builder main.go

Not related with this PR but required to compile:

• the function Unquote has been moved from arduino-cli to this repository

Should fix #336

[cmaglie]

@ArduinoBot build this please

[ArduinoBot]

✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/arduino-builder/arduino-builder-337.zip

ℹ️ To test this build:

1. Replace arduino-builder binary (you can find it where you installed the IDE) with the provided one