fix: Add shell detection for Windows NanoServer containers

[afnanashraf10]

Description

Adds automatic shell detection for Windows containers to support both ServerCore and NanoServer.

Problem

k9s currently hardcodes powershell for all Windows containers, which fails on NanoServer images that only include cmd.exe.

Error logs:

2025-10-19T21:47:11+04:00 ERR Exec failed error="exit status 1" 
cmd="[/usr/local/bin/kubectl exec ... -- powershell]"

Solution

Instead of hardcoding PowerShell, the fix uses shell detection:

cmd /c where powershell >nul 2>&1 && powershell || cmd

This command:

1. Checks if PowerShell exists using where powershell
2. Redirects output to avoid cluttering the console (>nul 2>&1)
3. Uses PowerShell if found (ServerCore) via &&
4. Falls back to cmd if not found (NanoServer) via ||

Benefits

• ✅ Maintains PowerShell preference for ServerCore (better shell features)
• ✅ Automatically works with NanoServer containers
• ✅ No breaking changes for existing users

Testing

• Tested with Windows ServerCore container - uses PowerShell
• Tested with Windows NanoServer container - uses cmd
• Verified shell access works in both scenarios

References

Fixes #1929

[afnanashraf10]

@derailed Would you be able to review this PR when you get a chance?

This fix addresses issue #1929 by implementing automatic shell detection for Windows containers to support both ServerCore (PowerShell) and NanoServer (cmd.exe) images, rather than hardcoding PowerShell which fails on NanoServer.

The solution uses a simple shell detection command that checks for PowerShell availability and falls back to cmd.exe if needed. I've tested it with both Windows container types and it's working as expected.

Thanks for maintaining k9s! 🐶