Enabling the rspamd debug log drowns out everything else and should be
selected explicitly as needed.
The external test does not require it and removing it makes it much
(~40.5%) faster, since it now does not block on terminal output anymore.
Before:
```
Benchmark 1: nix build .#hydraJobs.x86_64-linux.external-unstable --rebuild
Time (mean ± σ): 151.737 s ± 1.074 s [User: 0.310 s, System: 0.289 s]
Range (min … max): 150.321 s … 153.512 s 10 runs
```
After:
```
Benchmark 1: nix build .#hydraJobs.x86_64-linux.external-unstable --rebuild
Time (mean ± σ): 90.531 s ± 0.557 s [User: 0.054 s, System: 0.045 s]
Range (min … max): 89.579 s … 91.278 s 10 runs
```
cf6ef5e9ca