avm: Use #[cfg(unix)] when running UNIX-only functionality

[acheroncrypto]

Problem

After downloading the binary for the specific target, we change file permissions in UNIX to make the file executable:

anchor/avm/src/lib.rs

Lines 275 to 280 in 08110e9

	// Set file to executable on UNIX
	#[cfg(not(target_os = "windows"))]
	fs::set_permissions(
	bin_path,
	<fs::Permissions as std::os::unix::fs::PermissionsExt>::from_mode(0o775),
	)?;

While this technically works (because all non-Windows OS' we release binaries for are UNIX), it would still be better to use #[cfg(unix)] to make the intention clearer.

Summary of changes

Use #[cfg(unix)] when running UNIX-only functionality instead of #[cfg(not(target_os = "windows"))].

[vercel]

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