Fix unpack tar.gz archive with ADD instruction, issue #2409

[Greendor1234]

Fixes #2409

Description

This is a fix for the ADD instruction, as metioned in issue #2409 when used with multiple sources, specifically with tar.gz archive the build process didn't unpack the archive. I haven't written the unit test yet, but I checked the result by comparing two images (one builded with docker and the other one with kaniko) based on the Dockerfile described in the issue.

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.

Reviewer Notes

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

[aaron-prindle]

Thanks for the PR here @Greendor1234. Currently our CI/CD is erroring on this PR related to some lint errors:

Error: pkg/commands/add_test.go:39:33: `createFile` - `t` is unused (unparam)
func createFile(tempDir string, t *testing.T) error {
                                ^
Error: pkg/commands/add_test.go:54:50: `createTar` - `t` is unused (unparam)
func createTar(tempDir string, toCreate TarList, t *testing.T) error {
                                                 ^
Error: pkg/commands/add_test.go:75:2: ineffectual assignment to err (ineffassign)
	err = util.CreateTarballOfDirectory(filepath.Join(tempDir, toCreate.directory), tarFile)
	^
make: *** [Makefile:67: test] Error 1
FAILED /home/runner/work/kaniko/kaniko/scripts/../hack/linter.sh
Error: Process completed with exit code 2.

These need to be resolved to get the tests here all green, thanks

[aaron-prindle]

LGTM, thanks for the PR here @Greendor1234!