fix: use /etc/ssl/certs/ dir by default

[zhekazuev]

Fixes # in case of a bug fix, this should point to a bug and any other related issue(s)

Description

Submitter Checklist

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

• Includes unit tests
• Adds integration tests if needed.

See the contribution guide for more details.

Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

Examples of user facing changes:
- kaniko adds a new flag `--registry-repo` to override registry

[aaron-prindle]

Some context as to currently why this directory is /kaniko vs /etc:

#88