feat(cli): add support for 'pod install' in VM based environments

[thomasvidas]

Fixes #5069

Directly check for pod executable rather than just failing on not Mac OS. Allows for use of capacitor if running the cli from within a vm container like docker or vagrant.

[jcesarmobile]

The OS check as there more because of xcodebuild than because of pod install. I don’t think you can run xcodebuild on Linux. I have not tried the change on a vm, but it will probably fail because of that.
If we want to add this, then xcodebuild shouldn’t be run.

[thomasvidas]

I added a function to check if xcodebuild is installed. In the future we should refactor to have a "commandExists" function or something to check if a command is able to be run, but I think that is outside the scope of this PR

[jcesarmobile]

Added three comments.

We in fact have an isInstalled function already, so should use that instead of which (it uses which internally) and instead of having two functions for checking.

[thomasvidas]

Finally made the changes to this 😄

[jcesarmobile]

I've removed some unused imports, the platform check since the update file is the iOS file, so the platform is always iOS, and also moved the pod check right before running pod install since users might still want the Podfile to be updated despite they don't have CocoaPods installed in case they run update on windows/linux but build on mac without running update there