feat:[CI-15236]: Added IMAGE_TAR_PATH as output variable for the plugin.

[Anshika2203]

IMAGE_TAR_PATH is now accessible via a output variable and can also be used in STO steps as expression -
<+steps.STEP_ID.output.outputVariables.IMAGE_TAR_PATH>

[Ompragash]

add a validation check to ensure the directory for plugin_tar_path exists before use by making use of filepath.Dir, you can add this in if p.Build.TarPath != ""

also add some ut's to cover the same functionality for edge scenarios like in case of privileged and non-privileged k8s scenarios

Done

[Ompragash]

CI jobs are failing with the below error though it succeeds in local - remove the use /root dir in the tests, and may be rely on DRONE_WORKSPACE as it's common for both Drone and Harness or better use os.MkdirTemp

go: downloading golang.org/x/text v0.3.7
Warning: tar_path is empty
--- FAIL: TestTarPathValidation (0.00s)
    --- FAIL: TestTarPathValidation/invalid_path_no_permissions (0.00s)
        kaniko_test.go:239: Expected empty tar path, got non-empty string
FAIL
FAIL	github.com/drone/drone-kaniko	0.006s
ok  	github.com/drone/drone-kaniko/cmd/kaniko-acr	0.005s
ok  	github.com/drone/drone-kaniko/cmd/kaniko-docker	0.004s
?   	github.com/drone/drone-kaniko/cmd/kaniko-gar	[no test files]
ok  	github.com/drone/drone-kaniko/cmd/kaniko-ecr	0.011s
?   	github.com/drone/drone-kaniko/cmd/kaniko-gcr	[no test files]
ok  	github.com/drone/drone-kaniko/pkg/artifact	0.002s
ok  	github.com/drone/drone-kaniko/pkg/docker	0.004s
--- FAIL: TestWritePluginOutputFile (0.01s)
    --- FAIL: TestWritePluginOutputFile/invalid_output_path (0.00s)
        output_test.go:87: Expected error, got none
FAIL
FAIL	github.com/drone/drone-kaniko/pkg/output	0.011s

Fixed