Update plugin's `FZF_DEFAULT_COMMAND`

[unixorn]

Description

Update the plugin's FZF_DEFAULT_COMMAND per suggestion in #69. The plugin also only sets $FZF_DEFAULT_COMMAND` when it isn't already set so that any custom user settings don't get stepped on by the plugin.

Fixes #69.

Checklist

• [x] All new and existing tests pass.
• [ ] Rather than adding functions to fzf-zsh-plugin.zsh, I have created standalone scripts in bin so they can be used by non-ZSH users too.
• [ ] Any scripts added use #!/usr/bin/env interpreter instead of potentially platform-specific direct paths (#!/bin/sh is an ok exception)
• [ ] Scripts are marked executable
• [ ] Scripts do not have a language file extension unless they are meant to be sourced and not run standalone. No one should have to know if a script was written in bash, python, ruby or whatever. Not including file extensions makes it easier to rewrite the script in another language later without having to change every reference to the previous version.
• [ ] I have confirmed that the link(s) in my PR are valid.
• [x] I have read the CONTRIBUTING document.

License Acceptance

• [x] This repository is Apache version 2.0 licensed (some scripts may have alternate licensing inline in their code) and by making this PR, I am contributing my changes to the repository under the terms of the Apache 2 license.

[github-actions[bot]]

🦙 MegaLinter status: ⚠️ WARNING

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ COPYPASTE	jscpd	yes		no	0.97s
⚠️ REPOSITORY	checkov	yes		2	11.83s
✅ REPOSITORY	devskim	yes		no	1.67s
✅ REPOSITORY	dustilock	yes		no	0.0s
✅ REPOSITORY	gitleaks	yes		no	0.16s
✅ REPOSITORY	git_diff	yes		no	0.01s
✅ REPOSITORY	grype	yes		no	11.15s
⚠️ REPOSITORY	kics	yes		1	1.15s
✅ REPOSITORY	secretlint	yes		no	0.93s
✅ REPOSITORY	syft	yes		no	0.3s
✅ REPOSITORY	trivy	yes		no	3.81s
✅ REPOSITORY	trivy-sbom	yes		no	2.61s
✅ REPOSITORY	trufflehog	yes		no	7.88s

See detailed report in MegaLinter reports _Set VALIDATE_ALL_CODEBASE: true in mega-linter.yml to validate all sources, not only the diff_

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