assemblers/tar: fix compression

Commit 283281f broke compression by appending the argument last to the
tar command line. It needs to appear before the file.

Fix that and add a test.

[teg: add minor fix]

test/pipelines/f30-base.json

@@ -21,7 +21,8 @@
               "grub2-pc",
               "policycoreutils",
               "qemu-img",
-              "systemd"
+              "systemd",
+              "tar"
             ]
           }
         }