[Bug] az extension add fails when $ is present in username

> ### `az feedback` auto-generates most of the information requested below, as of CLI version 2.0.62

**Describe the bug**

> Exception:
> Traceback (most recent call last):
> File "C:\Program Files (x86)\Microsoft SDKs\Azure\CLI2\lib\distutils\util.py", line 202, in subst_vars
> return re.sub(r'\$([a-zA-Z_][a-zA-Z_0-9]*)', _subst, s)
> File "C:\Program Files (x86)\Microsoft SDKs\Azure\CLI2\lib\re.py", line 191, in sub
> return _compile(pattern, flags).sub(repl, string, count)
> File "C:\Program Files (x86)\Microsoft SDKs\Azure\CLI2\lib\distutils\util.py", line 199, in _subst
> return os.environ[var_name]
> File "C:\Program Files (x86)\Microsoft SDKs\Azure\CLI2\lib\os.py", line 669, in __getitem__
> raise KeyError(key) from None
> KeyError: 'Dlakin'
>
> During handling of the above exception, another exception occurred:
>
> Traceback (most recent call last):
> File "C:\Program Files (x86)\Microsoft SDKs\Azure\CLI2\lib\site-packages\pip\_internal\basecommand.py", line 228, in main
> status = self.run(options, args)
> File "C:\Program Files (x86)\Microsoft SDKs\Azure\CLI2\lib\site-packages\pip\_internal\commands\install.py", line 335, in run
> use_user_site=options.use_user_site,
> File "C:\Program Files (x86)\Microsoft SDKs\Azure\CLI2\lib\site-packages\pip\_internal\req\__init__.py", line 49, in install_given_reqs
> **kwargs
> File "C:\Program Files (x86)\Microsoft SDKs\Azure\CLI2\lib\site-packages\pip\_internal\req\req_install.py", line 748, in install
> use_user_site=use_user_site, pycompile=pycompile,
> File "C:\Program Files (x86)\Microsoft SDKs\Azure\CLI2\lib\site-packages\pip\_internal\req\req_install.py", line 961, in move_wheel_files
> warn_script_location=warn_script_location,
> File "C:\Program Files (x86)\Microsoft SDKs\Azure\CLI2\lib\site-packages\pip\_internal\wheel.py", line 216, in move_wheel_files
> prefix=prefix,
> File "C:\Program Files (x86)\Microsoft SDKs\Azure\CLI2\lib\site-packages\pip\_internal\locations.py", line 165, in distutils_scheme
> i.finalize_options()
> File "C:\Program Files (x86)\Microsoft SDKs\Azure\CLI2\lib\distutils\command\install.py", line 307, in finalize_options
> self.expand_basedirs()
> File "C:\Program Files (x86)\Microsoft SDKs\Azure\CLI2\lib\distutils\command\install.py", line 475, in expand_basedirs
> self._expand_attrs(['install_base', 'install_platbase', 'root'])
> File "C:\Program Files (x86)\Microsoft SDKs\Azure\CLI2\lib\distutils\command\install.py", line 469, in _expand_attrs
> val = subst_vars(val, self.config_vars)
> File "C:\Program Files (x86)\Microsoft SDKs\Azure\CLI2\lib\distutils\util.py", line 204, in subst_vars
> raise ValueError("invalid variable '$%s'" % var)
> ValueError: invalid variable '$'Dlakin''
>
> Command '['C:\\Program Files (x86)\\Microsoft SDKs\\Azure\\CLI2\\python.exe', '-m', 'pip', 'install', '--target', 'C:\\Users\\$Dlakin\\.azure\\cliextensions\\azure-devops', 'C:\\Users\\$Dlakin\\AppData\\Local\\Temp\\tmpbszvr1a1\\azure_devops-0.4.0-py2.py3-none-any.whl', '-vv', '--disable-pip-version-check', '--no-cache-dir']' returned non-zero exit status 2.
> Pip failed so deleting anything we might have installed at C:\Users\$Dlakin\.azure\cliextensions\azure-devops

**To Reproduce**
Try to install the az devops cli extension with $ character in username

**Expected behavior**
The devops cli extension is installed

**Environment summary**
Windows 10 - issue is down to $ in username

**Additional context**
n/a