Enable auto sync mode for Buildpacks artifacts

[briandealwis]

Fixes: #4651

Description

Change Buildpacks artifacts to default to automatic sync. This is a safe change as:

• The auto sync mechanism requires that the image contains buildpacks metadata defining the file types to be synced; if this metadata is not found (e.g., using something other than GCP Buildpacks) then no sync rules are defined and a full rebuild is triggered for each file change.
• The GCP Buildpacks are conservative in their listed file types.

The default can be avoided by defining a manual or infer rule:

   artifacts:
   - image: nodejs-hello-world
     buildpacks:
       builder: gcr.io/buildpacks/builder:v1
     sync:
      manual:
       # following rule never matches anything but prevents auto syncing
      - src: '...'
        dest: '...'

User facing changes (remove if N/A)

Buildpacks artifacts now default to enabling auto sync

[codecov]

Codecov Report

Merging #4656 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4656   +/-   ##
=======================================
  Coverage   73.20%   73.20%           
=======================================
  Files         339      339           
  Lines       13320    13322    +2     
=======================================
+ Hits         9751     9753    +2     
  Misses       2958     2958           
  Partials      611      611           

Impacted Files	Coverage Δ
pkg/skaffold/schema/defaults/defaults.go	91.00% <100.00%> (+0.09%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5a5c603...c2e37f1. Read the comment docs.

[gsquared94]

LGTM!