[Packaging] Update the bundled Python to 3.8

## Description

On windows, Azure CLI installed with `pip` (which uses the system Python 3.8) is faster than the one installed with MSI installer (which uses the bundled Python 3.6):

Installed with `pip`:
```
> Measure-Command {C:\Users\xxx\AppData\Local\Programs\Python\Python38\Scripts\az.bat version}

Days : 0
Hours : 0
Minutes : 0
Seconds : 1
Milliseconds : 231
Ticks : 12315568
TotalDays : 1.42541296296296E-05
TotalHours : 0.000342099111111111
TotalMinutes : 0.0205259466666667
TotalSeconds : 1.2315568
TotalMilliseconds : 1231.5568
```

Installed with MSI installer:
```
Measure-Command {& "C:\Program Files (x86)\Microsoft SDKs\Azure\CLI2\wbin\az.cmd" version}

Days : 0
Hours : 0
Minutes : 0
Seconds : 1
Milliseconds : 550
Ticks : 15507374
TotalDays : 1.7948349537037E-05
TotalHours : 0.000430760388888889
TotalMinutes : 0.0258456233333333
TotalSeconds : 1.5507374
TotalMilliseconds : 1550.7374
```

## Goal

We should evaluate whether to update the bundled/built-in Python of MSI installer (or maybe deb installer on Ubuntu) to 3.8 for performance improvement.