3052: clean exit

[KevinGimbel]

A wrong TERM value can cause k9s to crash, instead of crashing we can log a warning and silently set the variable for the duration of k9s' execution with os.Setenv. We recommend setting this variable in the README.md anyway (during the Preflight checks)

I've tested it with MacOS and would like to test on Linux as well.

How to test

Set TERM to xterm-ghostty

export TERM=xterm-ghostty

Run k9s, it panics:

k9s
panic: exit status 1

goroutine 1 [running]:
github.com/derailed/k9s/cmd.Execute()
        github.com/derailed/k9s/cmd/root.go:72 +0x80
main.main()

Run this branch, it works:

go run main.go
WARN: $TERM must be set to 'xterm-256color', got 'xterm-ghostty'
<k9s starts normally>

[KevinGimbel]

I accidentally added a WIP from another local branch to this PR, which I reverted again. I can also scrap and re-create the entire PR / branch if needed. :)