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

cli/exec/exec.go

@@ -72,9 +72,6 @@ func execWithAxis(c *cli.Context, axis matrix.Axis) error {
 	metadata := metadataFromContext(c, axis)
 	environ := metadata.Environ()
 	var secrets []compiler.Secret
-	for k, v := range metadata.EnvironDrone() {
-		environ[k] = v
-	}
 	for key, val := range metadata.Job.Matrix {
 		environ[key] = val
 		secrets = append(secrets, compiler.Secret{