nixos/acme: Fix rate limiting of selfsigned services
nixos/acme: Fix rate limiting of selfsigned services

Closes NixOS/nixpkgs#147348

I was able to reproduce this intermittently in the
test suite during the tests for HTTPd. Adding
StartLimitIntervalSec=0 to disable rate limiting
for these services works fine. I added it anywhere
there was a ConditionPathExists.

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