Re-enable single server mode in Drone CI

Drone CI comes since version 1.5.0 with multi-server mode (in which builds are processed by external CI runners) activated by default.

This can be problematic if you’re used to drone running in single server mode. If single-server mode isn’t explicitly disabled and no runners are configured it just causes your builds to be stuck with a pending status.

It took me quite a while to figure why drone suddenly stopped processing builds (to be fair I updated without reading the changelog).

Luckily there is an easy solution to re-enable single-server mode.

Just add the environment variable DRONE_AGENTS_DISABLED=true to your drone instance and it should process builds again like it did before.