Hardcode the paths to Windows PowerShell and CMD

Occasionally, we get users with corrupt PATH environment variables: they
can't lauch PowerShell, because for some reason it's dropped off their
PATH. We also get users who have stray applications named
`powershell.exe` just lying around in random system directories.

We can combat both of these issues by simply hardcoding where we expect
PowerShell and CMD to live. %SystemRoot% was chosen over %WINDIR%
because apparently (according to Stack Overflow), SystemPath is
read-only and WINDIR isn't.

Refs #6039, #4390, #4228, #4682 (stray powershell.exe).
This may impact #6082 (as it might be another "stray file" incident).

src/cascadia/TerminalApp/defaults.json

@@ -30,7 +30,7 @@
         {
             "guid": "{61c54bbd-c2c6-5271-96e7-009a87ff44bf}",
             "name": "Windows PowerShell",
-            "commandline": "powershell.exe",
+            "commandline": "%SystemRoot%\\System32\\WindowsPowerShell\\v1.0\\powershell.exe",
             "icon": "ms-appx:///ProfileIcons/{61c54bbd-c2c6-5271-96e7-009a87ff44bf}.png",
             "colorScheme": "Campbell",
             "antialiasingMode": "grayscale",
@@ -49,7 +49,7 @@
         {
             "guid": "{0caa0dad-35be-5f56-a8ff-afceeeaa6101}",
             "name": "Command Prompt",
-            "commandline": "cmd.exe",
+            "commandline": "%SystemRoot%\\System32\\cmd.exe",
             "icon": "ms-appx:///ProfileIcons/{0caa0dad-35be-5f56-a8ff-afceeeaa6101}.png",
             "colorScheme": "Campbell",
             "antialiasingMode": "grayscale",