tests: don't use absolute paths when installing binaries in TEST-58-REPART

Let image_install figures the binary paths out itself as not all distributions
ship the mkfs.* tools in the same location. For example openSUSE ships
mksquashfs in /usr/bin/.

test/TEST-58-REPART/test.sh

@@ -15,8 +15,8 @@ test_append_files() {
         instmods dm_verity =md
         instmods erofs
         generate_module_dependencies
-        image_install -o /sbin/mksquashfs
-        image_install -o /bin/mkfs.erofs
+        image_install -o mksquashfs
+        image_install -o mkfs.erofs
     fi
 
     inst_binary mcopy