Don't override existing env

This would interfere with how the k8s stack operates. See comment.

clicommand/bootstrap.go

@@ -408,7 +408,20 @@ var BootstrapCommand = cli.Command{
 			if err != nil {
 				return fmt.Errorf("error connecting to kubernetes runner: %w", err)
 			}
+
+			// Set our environment vars based on the registration response.
+			// But note that the k8s stack interprets the job definition itself,
+			// and sets a variety of env vars (e.g. BUILDKITE_COMMAND) that
+			// *could* be different to the ones the agent normally supplies.
+			// A prime example is: the command container could be passed a
+			// specific BUILDKITE_COMMAND that is computed from the command+args
+			// podSpec attributes (in the kubernetes "plugin"), instead of the
+			// "command" attribute of the step.
+			// So we should skip setting any vars that are already set.
 			for n, v := range env.FromSlice(regResp.Env).Dump() {
+				if _, set := os.LookupEnv(n); set {
+					continue
+				}
 				if err := os.Setenv(n, v); err != nil {
 					return err
 				}