spread tests: unset SNAPCRAFT_BUILD_ENVIRONMENT=host for lxd tests

New sanity checks detect the mismatch of --use-lxd and
SNAPCRAFT_BUILD_ENVIRONMENT=host set by the top tree.

The LXD tests which use `sudo` already lose the environment,
so we do not need to unset those.

Signed-off-by: Chris Patterson <chris.patterson@canonical.com>

tests/spread/build-providers/lxd-basic/task.yaml

@@ -10,6 +10,10 @@ prepare: |
 
 restore: |
   cd "$SNAP_DIR"
+
+  # Unset SNAPCRAFT_BUILD_ENVIRONMENT=host.
+  unset SNAPCRAFT_BUILD_ENVIRONMENT
+
   snapcraft clean --use-lxd
   rm -f ./*.snap
 
@@ -20,6 +24,9 @@ restore: |
 execute: |
   cd "$SNAP_DIR"
 
+  # Unset SNAPCRAFT_BUILD_ENVIRONMENT=host.
+  unset SNAPCRAFT_BUILD_ENVIRONMENT
+
   snapcraft --use-lxd
   sudo snap install make-hello_*.snap --dangerous
   [ "$(make-hello)" = "hello world" ]

tests/spread/build-providers/lxd-type-base/task.yaml

@@ -6,12 +6,18 @@ environment:
 restore: |
   cd "$SNAP_DIR"
 
+  # Unset SNAPCRAFT_BUILD_ENVIRONMENT=host.
+  unset SNAPCRAFT_BUILD_ENVIRONMENT
+
   snapcraft clean --use-lxd
   rm -f ./*.snap
 
 execute: |
   cd "$SNAP_DIR"
 
+  # Unset SNAPCRAFT_BUILD_ENVIRONMENT=host.
+  unset SNAPCRAFT_BUILD_ENVIRONMENT
+
   snapcraft pull --use-lxd
 
   /snap/bin/lxc start snapcraft-core18