Cache the result of environment checks

[rosen-vladimirov]

Is your feature request related to a problem? Please describe.
Currently most of the CLI commands check the environment, which takes a lot of time. For example, when tns run ios is executed, it takes between 6 and 12 seconds on my machine to validate the environment. When the command is stopped and run again, it checks the environment again.

Describe the solution you'd like
Cache the result of the first check and if the conditions are not changed, use the cache result. For example, in case the value of ANDROID_HOME is changed, the result should not be used from the cache.
In case tns doctor is executed, skip the cache and always check the results.

Describe alternatives you've considered
No.

Additional context