Fix image override bug for mutate-image in config

[dalbar]

Changes

Minimal example to reproduce:

package config

import (
	"os"
	"testing"
)

func TestOverrideBug(t *testing.T) {
	cfg := NewDefaultConfig()

	os.Setenv("GIT_CONTAINER_IMAGE", "git-image")
	cfg.SetConfigFromEnv()

	if cfg.GitContainerTemplate.Image != "git-image" {
		t.Fatal("override did not set git image")
	}

	os.Setenv("MUTATE_IMAGE_CONTAINER_TEMPLATE", `{"image":"mutate-image"`)
	cfg.SetConfigFromEnv()

	if cfg.GitContainerTemplate.Image != "git-image" {
		t.Fatalf("mutate image overrides git-template")
	}
}

Fixes bug by overriding the right entry of config instead of the GitContainerTemplate.
It additionally adds previously missing unit tests to make sure that override for mutate-image is sound.

Submitter Checklist

• Includes tests if functionality changed/was added
• Includes docs if changes are user-facing
• Set a kind label on this PR
• Release notes block has been filled in, or marked NONE

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Release Notes

Fixes override bug for `pkg/config`: Setting the environment variabel `MUTATE_IMAGE_CONTAINER_TEMPLATE` now works as intended and does not override `config.GitContainerTemplate`.

/kind bug

[SaschaSchwarze0]

/lgtm
/approve

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: SaschaSchwarze0

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[otaviof]

/lgtm