It's not stuck on "Releasing lock", that process has exited: Main PID: 180885 (code=exited, status=0\/SUCCESS)
The code actually does nothing after printing that, the script just exits which automatically frees the lock (source)

Based on the CGroup content I think it's stuck on reloading Nginx though I don't understand how that would block or why it's doing that, as Nginx is supposed to reload itself through nginx-config-reload.service.
What's the content of /nix/store/vw98jrrshrz371az32ssbcwrr3bz2fqs-acme-postrun?
What's the value of services.nginx.enableReload? Did you add nginx to the cert's reloadServices?

#!/nix/store/0irlcqx2n3qm6b1pc9rsd2i8qpvcccaj-bash-5.2p37/bin/bash
cd /var/lib/acme/spam.lossy.network
if [ -e renewed ]; then
  rm renewed
  systemctl reload nginx

  
fi

#!/nix/store/0irlcqx2n3qm6b1pc9rsd2i8qpvcccaj-bash-5.2p37/bin/bash
cd /var/lib/acme/spam.lossy.network
if [ -e renewed ]; then
  rm renewed
  systemctl reload nginx # <-- stuck here
fi

  security.acme.certs."spam.${config.networking.domain}" = {
    postRun = ''
      systemctl reload nginx
    '';
  };

Thread 1 (Thread 0x7f7f4c1c5680 (LWP 180909) "systemctl"):
#0  0x00007f7f4c50963c in ppoll () from target:/nix/store/pacbfvpzqz2mksby36awvbcn051zcji3-glibc-2.40-36/lib/libc.so.6
No symbol table info available.
#1  0x00007f7f4c82270b in ppoll_usec () from target:/nix/store/ivqjhj99firnjq7gp14qf35821viwi5m-systemd-256.7/lib/systemd/libsystemd-shared-256.so
No symbol table info available.
#2  0x00007f7f4c89e33a in bus_poll () from target:/nix/store/ivqjhj99firnjq7gp14qf35821viwi5m-systemd-256.7/lib/systemd/libsystemd-shared-256.so
No symbol table info available.
#3  0x00007f7f4c89e6c5 in sd_bus_wait () from target:/nix/store/ivqjhj99firnjq7gp14qf35821viwi5m-systemd-256.7/lib/systemd/libsystemd-shared-256.so
No symbol table info available.
#4  0x00007f7f4c6c41b9 in bus_wait_for_jobs () from target:/nix/store/ivqjhj99firnjq7gp14qf35821viwi5m-systemd-256.7/lib/systemd/libsystemd-shared-256.so
No symbol table info available.
#5  0x00005641d8690e2c in verb_start ()
No symbol table info available.
#6  0x00005641d8672bea in main ()