'az pipelines variable update' fails on special characters (^, >)

## Describe the bug

`az pipelines variable update` has difficulties dealing with some special characters:

### works
```
$key="semver"
$value="~1.0.0"

az pipelines variable update --project $projectName --pipeline-name $pipelineName --name --value $key $value
```

### errors
```
$key="semver"
$value="^1.0.0" # caret (^) will be ignored in the UI frontend
$value=">=1.0.0" # error: (>) leads to an error

az pipelines variable update --project $projectName --pipeline-name $pipelineName --name --value $key $value
```

## Environment Summary
```
Windows-10-10.0.19041-SP0
Python 3.6.6
Installer: MSI

azure-cli 2.3.1

Extensions:
aks-preview 0.4.66
azure-devops 0.18.0
azure-iot 0.9.3
storage-preview 0.2.12

```