tests: install ubuntu image snap to build uc18 and uc2* (#12891)

* Install ubuntu image snap to build uc18 and uc2*

This is needed because currently ubuntu-image is failing to build the
ubuntu core image with the following error:

runtime: goroutine stack exceeds 1000000000-byte limit
runtime: sp=0xc0204963b0 stack=[0xc020496000, 0xc040496000]
fatal error: stack overflow

* Use the ubuntu-image snap

tests/lib/prepare.sh

@@ -985,8 +985,14 @@ setup_reflash_magic() {
         snap install ubuntu-image --channel="$UBUNTU_IMAGE_SNAP_CHANNEL" --classic
     else
         # shellcheck source=tests/lib/image.sh
-        . "$TESTSLIB/image.sh"
-        get_ubuntu_image
+        #. "$TESTSLIB/image.sh"
+        #get_ubuntu_image
+        # TODO: revert this once ubuntu-image is fixed
+        # Currently it is failing with
+        # runtime: goroutine stack exceeds 1000000000-byte limit
+        # runtime: sp=0xc0204963b0 stack=[0xc020496000, 0xc040496000]
+        # fatal error: stack overflow
+        snap install ubuntu-image --channel="$UBUNTU_IMAGE_SNAP_CHANNEL" --classic
     fi
 
     # needs to be under /home because ubuntu-device-flash
@@ -1179,7 +1185,9 @@ EOF
 
         EXTRA_FUNDAMENTAL="$EXTRA_FUNDAMENTAL --snap ${IMAGE_HOME}/${BASE}.snap"
     fi
-    local UBUNTU_IMAGE="$GOHOME"/bin/ubuntu-image
+    # TODO: revert this when ubuntu-image issue is fixed
+    #local UBUNTU_IMAGE="$GOHOME"/bin/ubuntu-image
+    UBUNTU_IMAGE=/snap/bin/ubuntu-image
     if os.query is-core16 || os.query is-arm; then
         # ubuntu-image on 16.04 needs to be installed from a snap
         UBUNTU_IMAGE=/snap/bin/ubuntu-image