MacosKeychain.cs

using System.Collections;
using UnityEngine;

public class MacosKeychain
{

	public static string GetKeychainPassword(string accountName,string serviceName)
	{
		var value = ShellHelper.Bash($"security find-generic-password -a {accountName} -s {serviceName} -w -g");
		return value.TrimEnd( '\r', '\n' );
	}
}

set_android_passwords.sh

#!/bin/bash

echo
echo "Change android keystore passwords in keychain"
echo
echo "Enter keystore password"
security add-generic-password -U -T "" -a unity_android_keystore_password -s unity_android_keystore -w

echo
echo "Enter keyalias password"
security add-generic-password -U -T "" -a unity_android_keyalias_password -s unity_android_keyalias -w

echo "keychain passwords changed!"
echo 

SetAndroidPasswordsFromKeychain.cs

using UnityEditor;
using UnityEditor.Build;
using UnityEditor.Build.Reporting;

public class SetAndroidPasswordsFromKeychain : IPreprocessBuildWithReport{
	
	public int callbackOrder { get; }
	public void OnPreprocessBuild(BuildReport report)
	{
		if (report.summary.platform == BuildTarget.Android)
		{
			PlayerSettings.keyaliasPass = MacosKeychain.GetKeychainPassword("unity_android_keyalias_password","unity_android_keyalias");
			PlayerSettings.keystorePass = MacosKeychain.GetKeychainPassword("unity_android_keystore_password","unity_android_keystore");
		}
	}
}

ShellHelper.cs

using System.Diagnostics;

public static class ShellHelper
{
	public static string Bash(string cmd)
	{
		var escapedArgs = cmd.Replace("\"", "\\\"");

		var process = new Process()
		{
			StartInfo = new ProcessStartInfo
			{
				FileName = "/bin/bash",
				Arguments = $"-c \"{escapedArgs}\"",
				RedirectStandardOutput = true,
				UseShellExecute = false,
				CreateNoWindow = true,
			}
		};
		process.Start();
		string result = process.StandardOutput.ReadToEnd();
		process.WaitForExit();
		return result;
	}
}