Replace DRONE_ with CI_ variables in pipeline steps (#427)

Dropped support for `DRONE_*` environment variables in pipeline steps. Pipeline meta-data can be accessed with `CI_*` variables.
  - `CI_*` prefix replaces `DRONE_*`
  - `CI` value is now `woodpecker`
  - `DRONE=true` has been removed

pipeline/pipeline.go

@@ -2,6 +2,7 @@ package pipeline
 
 import (
 	"context"
+	"strings"
 	"time"
 
 	"github.com/rs/zerolog/log"
@@ -127,6 +128,13 @@ func (r *Runtime) exec(proc *backend.Step) error {
 		}
 	}
 
+	// TODO: using DRONE_ will be deprecated with 0.15.0. remove fallback with following release
+	for key, value := range proc.Environment {
+		if strings.HasPrefix(key, "CI_") {
+			proc.Environment[strings.Replace(key, "CI_", "DRONE_", 1)] = value
+		}
+	}
+
 	if err := r.engine.Exec(r.ctx, proc); err != nil {
 		return err
 	}