Build process: should no longer build Ride on mac1 - now use mac2 and possibly move to mac4

[dyaandys]

We currently build Ride for macOS on mac1 - which is running High Sierra (to allow us to support 18.0 and earlier).

The version of Electron that we want to use requires a more recent version of node - so we will move the build process to mac2.

For now just master, but we may need to do the same for earlier versions of Ride.

We may also want to move to building on mac4 as it is an ARM-based mac.

Related to #1316

[abrudz]

@dyaandys Should this be in Mantis or does it require changes to Ride's source?

[dyaandys]

This is best off being here I think .. the changes are probably needed only in Jenkins, but we ought to double check. It's certainly not anything in svn that needs to be changed,

[dyaandys]

Now building Ride-4.6 on mac4. Given that there will not be a 20.0 for mac/Intel, that should not be a problem.