Never select PowerShell if it isn't available

[cottsay]

Regardless of our heuristics for determining if PowerShell is in use, we should never attempt to use PowerShell as the primary shell if it isn't installed.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (2724c2c) to head (c890329).

Files with missing lines	Patch %	Lines
colcon_powershell/shell/powershell.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master     #42   +/-   ##
======================================
  Coverage    0.00%   0.00%           
======================================
  Files           2       2           
  Lines          81      83    +2     
======================================
- Misses         81      83    +2     

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[MatthijsBurgh]

@cottsay This PR is probably a situation which showcases the heuristics aren't robust.

[cottsay]

... the heuristics aren't robust.

Yeah, agreed, we were discussing the same in #39. I don't think the environment variables change enough when using pwsh for us to reliably determine if we're actually running under PowerShell.

This PR isn't a solution in the slightest. I'm open to suggestions on how we might more reliably tell if we're invoked from pwsh on non-Windows platforms.