Use file sync only

hi! Thank you for this wonderful product, it looks very promising!
A small question: is it possible to use JUST [file sync](https://skaffold.dev/docs/filesync/)?

I think I did something wrong here, but I can't figure out what exactly.

### Expected behavior
I modify a file and, according to the [doc](https://skaffold.dev/docs/filesync/), Skaffold creates a tar file with changed files that match the sync rules. This tar file is sent to and extracted on the corresponding containers.

### Actual behavior
Skaffold builds an image and tries to redeploy the whole app
the Skaffold output:
```
Generating tags...
- my-image -> my-image:0.1.9-01c818c
Checking cache...
- my-image: Not found. Building
Starting build...
Building [my-image]...
Target platforms: [linux/amd64]

Build [my-image] succeeded
Tags used in deployment:
- my-image -> my-image:0.1.9-01c818c@sha256:ec687110b5af7458c733a746035654fe9f95c3d15856a9e924212502de472a66
Starting deploy...
Waiting for deployments to stabilize...
Deployments stabilized in 731.038406ms

```

### Information

- Skaffold version:
v2.9.0

- Operating system: Linux 5.15.0-88-generic 20.04.1-Ubuntu
- Installed via: skaffold.dev
- Contents of skaffold.yaml:

```yaml

apiVersion: skaffold/v4beta8
kind: Config
metadata:
name: myapp
build:
local: {}
artifacts:
- image: my-image
context: .
sync:
manual:
- src: "src/"
dest: "/app/src"
- src: "__version__.py"
dest: "/app/__version__.py"
custom:
buildCommand:
"make docker-build-dev"
tagPolicy:
customTemplate:
template: 0.1.9-01c818c
manifests:
rawYaml:
- deployment.yaml

deploy:
kubectl:
defaultNamespace: mynamespace
flags:
global:
- --context=some-cluster

```

### Steps to reproduce the behavior

1. a clonable repository with the sample skaffold project
2. `skaffold dev`