automatically start/upgrade buildkitd

- Automatically start a buildkit daemon if no BUILDKIT_HOST is
  provided (and if not already started)
- Customization of BUILDKIT_HOST is still possible, just like before
- Automatically upgrade the managed daemon to the version used by dagger
  if necessary
- Add CI test to make sure the managed buildkit and the vendored
  buildkit versions match

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

Makefile

@@ -19,10 +19,17 @@ cuefmt:
 	@(cue fmt -s ./examples/*)
 
 .PHONY: lint
-lint: cuefmt
+lint: cuefmt check-buildkit-version
 	golangci-lint run
 	@test -z "$$(git status -s . | grep -e "^ M"  | grep .cue | cut -d ' ' -f3 | tee /dev/stderr)"
 
+.PHONY: check-buildkit-version
+check-buildkit-version:
+	@test \
+		"$(shell grep buildkit ./go.mod | cut -d' ' -f2)" = \
+		"$(shell grep ' = "v' ./pkg/buildkitd/buildkitd.go | sed -E 's/^.*version.*=.*\"(v.*)\"/\1/' )" \
+		|| { echo buildkit version mismatch go.mod != pkg/buildkitd/buildkitd.go ; exit 1; }
+
 .PHONY: integration
 integration: dagger-debug
 	# Self-diagnostics