Fix issue with ps format options

[SamuelSarle]

#105 introduced a call to ps -o exe= but exe isn't a standard option and not found in some versions of ps.

Changing exe to command fixes this issue. The command option is found in Linux, MacOS, FreeBSD, OpenBSD man pages for ps and should therefore be available.

Tested on NixOS

Fixes #109

[avant1]

This patch fixes aforementioned issue for my environment:

fish, version 3.2.2
tmux 3.0a
Pop!_OS 20.04 LTS

[jaideeprana]

Hi,

Any ETA on when this PR can be merged?

[AaronLasseigne]

This fixed it for me.

fish 3.3.0
tmux 3.2a
macOS 11.4 

[jaideeprana]

@AaronLasseigne Since it's working when can this be merged to the master?

[franciscolourenco]

Hey everyone, sorry for the delay on merging this PR.

@SamuelSarle would it make sense to call string match directly instead of using a pipe like in https://github.com/franciscolourenco/done/pull/111/files#diff-3e8fa8c31a036bc177a78e46da54de1c74b7da15ce98914fddfba37c1f444d48R114

[SamuelSarle]

@franciscolourenco Yes, it would be better. Pushed a commit.

[franciscolourenco]

Thank you for the contribution!