Remove deprecated reading of env vars for determining diff program

[Ahajha]

My environment for whatever reason has DISPLAY=1, and so in diff mode I always get this warning. A whole bunch of this has been deprecated since 2019, just delete it so I can silence it.

[gemini-code-assist]

Code Review

This pull request simplifies the diff command selection by removing deprecated environment variables and setting the default diff command to 'diff --unified'. However, removing the Windows fallback to 'FC' will break diffing on Windows systems without bash and diff installed, so this fallback should be restored. Additionally, the documentation comment for the Find function is now outdated and should be updated, and the parentheses around its single return type should be removed to adhere to idiomatic Go formatting.

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[gemini-code-assist]

Removing the Windows fallback to FC will break buildifier -mode=diff on Windows environments that do not have bash and diff installed.

When d.Cmd is set to "diff --unified", the Differ.run method attempts to execute it via /usr/bin/env bash, which is not available on standard Windows systems. Keeping the FC fallback for Windows ensures that diffing continues to work out-of-the-box on Windows.

Note: You will also need to add "runtime" back to the imports list at the top of this file.

	if runtime.GOOS == "windows" {
		d.Cmd = "FC"
	} else {
		d.Cmd = "diff --unified"
	}

References

1. When modifying code, ensure that changes are consistent with previous behavior, especially if the previous behavior was intentional.

[Ahajha]

Hmm, I'm not sure about this one. #520 marked this path as deprecated too, but I'm wondering the reason for making the sole Windows path here deprecated.

[Ahajha]

@vladmos Could you please TAL?