Set BUILDAH_ISOLATION env var

By default, `buildah` creates its working containers using kernel
namespaces.  When it is running in a container itself, this is not
possible, so it must use a plain `chroot`.  Setting this environment
variable will ensure that it does this by default, without having to
explicitly pass `--isolation chroot` on the command line.

Containerfile

@@ -33,3 +33,5 @@ RUN --mount=type=cache,target=/var/cache \
 	&& :
 
 COPY --from=build /usr/local/bin/genimage /usr/local/bin/genimage
+
+ENV BUILDAH_ISOLATION=chroot