Mention KREW_ROOT on instruction

[JohnTitor]

Related issue: #767
The instruction on

krew/hack/krew.yaml

Lines 13 to 16 in 93827db

	* macOS/Linux:
	- Add the following to your ~/.bashrc or ~/.zshrc:
	export PATH="${KREW_ROOT:-$HOME/.krew}/bin:$PATH"
	- Restart your shell.

mentions KREW_ROOT but one on cmd/krew/cmd/internal/setup_check.go doesn't.
This tweaks that instruction to mention KREW_ROOT there too.
The fish command is a bit long as there's no simple alternative to the unset variables on fish AFAIK.

[linux-foundation-easycla]

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• ✅ login: JohnTitor / name: Yuki Okushi (1221749)

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[JohnTitor]

/assign @ahmetb

[ahmetb]

this is not worth changing just because of krew completion (which we've removed since) but I think it's ok since the install guide already offers this functionality https://krew.sigs.k8s.io/docs/user-guide/setup/install/ (though the zsh cmd doesn't use the KREW_ROOT as you noted here).

[ahmetb]

/lgtm
/approve
Thanks!

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