Treat DISABLE_AUTOBUILD as number

[FluidSense]

!!"0" equals to true which does not feel correct when trying to disable the flag.
unset OPENCV4NODEJS_DISABLE_AUTOBUILD is the only way to disable it as it stands now.

[FluidSense]

Checks failing, I suspect these are not due to my change? If they are, please highlight how so I can try to fix it!

3397 passing (33s)
  52 pending
  3 failing
  1) cv
       OpenCV version should match:
      AssertionError: expected '' to equal '3.4.6'
      + expected - actual
      +3.4.6
      
      at Context.it (tests\index.test.js:80:64)
  2) cv
       io
         VideoCapture
           set
             should set properties:
      AssertionError: expected 1000 to equal 1001
      + expected - actual
      -1000
      +1001
      
      at Context.it (tests\io\VideoCaptureTests.js:53:50)
  3) cv
       io
         VideoCapture
           setAsync
             should set properties:
      Uncaught AssertionError: expected 1000 to equal 1001
      + expected - actual
      -1000
      +1001
      
      at cap.setAsync (tests\io\VideoCaptureTests.js:62:52)
npm ERR! code ELIFECYCLE
npm ERR! errno 3
npm ERR! [email protected] test-appveyor: `set APPVEYOR_BUILD=true && mocha --timeout 30000 ./tests/index.test.js`
npm ERR! Exit status 3
npm ERR! 
npm ERR! Failed at the [email protected] test-appveyor script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.
npm ERR! A complete log of this run can be found in:
npm ERR!     C:\Users\appveyor\AppData\Roaming\npm-cache\_logs\20