snapcraft.yaml: use build-base instead of base

The snapd snap should not have a base. This will currently break building the
snap until https://bugs.launchpad.net/snapcraft/+bug/1857019 is fixed however.

Signed-off-by: Ian Johnson <ian.johnson@canonical.com>

build-aux/snap/snapcraft.yaml

@@ -12,7 +12,7 @@ description: |
 adopt-info: snapd
 # the base is only needed here for snapcraft to check the ELF sections of
 # binaries inside the snap, which is an unnecessary check for snapcraft itself
-base: core
+build-base: core
 grade: stable
 license: GPL-3.0