How to edit /etc/rc.local when raspbian shuts down immediately

I dun screwed up. I have a raspberry pi zero W and I have a project that needs to be as low power as possible. For my use case it's ok for the raspi to be powered down for long periods as it just needs to send a wifi message in response to a button being triggered. So I edited /etc/rc.local to run my python script on startup, and (rather stupidly, apparently) put call("sudo shutdown -h now", shell=True) at the end of my python script. The result is the pi starts up when powered (or when pins 5-6 are shorted), sends the message, and shuts itself down.

But-- now I can't make the pi stay booted up so that I can make an update to the code.

Is there any way to edit either my python script (in /home/somedir) or /etc/rc/local now? I tried putting the sd card into a different computer, but it appears my files are embedded in .tar.xz files. Can anyone help?