Fix creating debian packages with latest debhelpers

Getting this error otherwise:

```shell
+ go build -mod=vendor -v -o obj-x86_64-linux-gnu/bin/tkn -ldflags -X github.com/tektoncd/cli/pkg/cmd/version.clientVersion=0.25.0 ./cmd/tkn
build flag -mod=vendor only valid when using modules
make[1]: *** [debian/rules:17: override_dh_auto_build] Error 1
make[1]: Leaving directory '/<<PKGBUILDDIR>>'
make: *** [debian/rules:14: build] Error 2
dpkg-buildpackage: error: debian/rules build subprocess returned exit status 2
```

Signed-off-by: Chmouel Boudjnah <[email protected]>

tekton/debbuild/control/rules

@@ -11,7 +11,7 @@ endif
 BUILD_DATE := $(BUILD_DATE:+0000=Z)
 
 %:
-	dh $@
+	dh $@ --builddirectory=_build --buildsystem=golang
 
 override_dh_auto_build:
 	@set -x ; export XDG_CACHE_HOME=/tmp/cache ; \