Rekey loop adds multiple -o flags

[luishfonseca]

Right now, every iteration of the rekey loop adds a -o flag to DEFAULT_DECRYPT (line 154), causing rage to fail on the second secret with:

error: the argument '--output <OUTPUT>' cannot be used multiple times

This probably caused #245

I think have it working correctly on my fork https://github.com/luishfonseca/agenix/blob/main/pkgs/agenix.sh