distro/rhel85: tar-installer -> image-installer

Renamed tar-installer to image-installer.
This is a more appropriate name:
- It disassociates the image type from the "tar" image type. The two
  should not be perceived to be connected.
- It's more descriptive. The format of the payload (tar) isn't relevant
  to the purpose of the image type.

test/data/manifests/rhel_85-x86_64-image_installer-boot.json

@@ -2,7 +2,7 @@
   "compose-request": {
     "distro": "rhel-85",
     "arch": "x86_64",
-    "image-type": "tar-installer",
+    "image-type": "image-installer",
     "repositories": [
       {
         "name": "baseos",