tests: install: Fix iso-image-installer test.

This is a follow-up of: 45eac6cdf5.
It fixes the following error:

+ mkfs.ext4 -L my-root /dev/vda2
sh: line 12: mkfs.ext4: command not found

* gnu/tests/install.scm (%test-iso-image-installer): Add e2fsprogs to the
appended packages.

gnu/tests/install.scm

@@ -471,6 +471,8 @@ reboot\n")
     (mlet* %store-monad ((images (run-install
                                   %minimal-os-on-vda
                                   %minimal-os-on-vda-source
+                                  #:packages
+                                  (list e2fsprogs)
                                   #:script
                                   %simple-installation-script-for-/dev/vda
                                   #:installation-image-type