fix: speed up the prepare process

[Fatme]

Currently CLI executes which pod and which xcodeproj every time when pod install is executed e.g on every native change. Also, CLI checks if env is correctly setup on every "prepare" related command - e.g prepare, build, run, debug and test. However, which xcodeproj
and pod --version are included in the checks for correctly setup env. So these checks shouldn't be executed before each pod install as it aditionally slow down the prepare process.

Currently CLI checks if xcproj executable is available on user's machine in case when CocoaPods version is lower than 1.0.0 and Xcode's version is greater than 7.3 and uses it to convert the project to ASCII plist format. This logic was needed as there is an issue in case when CocoaPods is with version lower than 1.0.0 and Xcode is with version >= 7.3.0 - all .pbxproject files are converted to XML plst format. However, CLI requires Xcode 10 as min supported version and based on this blog post it seems it is not possible to use CocoaPods with version lower than 1.0.0 and Xcode 10+.

Rel to: #4971

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/nativescript-cli/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• Tests for the changes are included.

What is the current behavior?

What is the new behavior?

Fixes/Implements/Closes #[Issue Number].

[Fatme]

test cli-templates

[rosen-vladimirov]

test cli-prepare cli-device cli-run

[rosen-vladimirov]

Really awesome work!
Merge after green build

[rosen-vladimirov]

test cli-device

[miroslavaivanova]

test cli-device