You need to republish an app. You want to sign the app with the android-build-key.pem key file, but you realized you are not able to open the .pem key file (e.g. with Keychain Access).
You do not need to "open" the .pem key file to republish the app. You only need to upload it to your release submission as per the instructions in Google Play Store Requires New AAB Format For BiznessApps App Submissions.