Properly tag images with digest when using helm

[michaelbeaumont]

Fixes #2624 again as reintroduced by #2624

Description

User facing changes

Include digest in image tag when deploying with helm

Before

Skaffold did not output the digest, leading to outdated deployments

After

Ensure that the correct digest of an image is used.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
• Mentions any output changes.
• Adds documentation as needed: user docs, YAML reference, CLI reference.
• Adds integration tests if needed.

See the contribution guide for more details.

Double check this list of stuff that's easy to miss:

• If you are adding a example to the examples dir, please copy them to integration/examples
• Every new example added in integration/examples dir, should be tested in integration test

Reviewer Notes

• The code flow looks good.
• Unit test added.
• User facing changes look good.

Release Notes

[codecov]

Codecov Report

Merging #2956 into master will increase coverage by 0.01%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
pkg/skaffold/deploy/helm.go	77.25% <100%> (+0.41%)	⬆️

[tejal29]

/cc @dgageot