CI: sign plugin during release (#1039)

* Add grafana toolkit package

* CI: skip adding binaries to release branch

* CI: test signing plugin

* CI: skip branch build for test releases

* CI: do not use ssh key for fetching

* CI: get plugin in a separate image to avoid git LFS error

* CI: fix attaching workspace

* CI: fix attaching dir

* CI: adjust release process

* CI: clean up

package.json

@@ -32,6 +32,7 @@
     "@emotion/core": "10.0.27",
     "@grafana/data": "7.0.1",
     "@grafana/runtime": "7.0.1",
+    "@grafana/toolkit": "^7.1.5",
     "@grafana/ui": "7.0.1",
     "@popperjs/core": "2.4.0",
     "@types/classnames": "2.2.9",