ci: introduce 'drone-skip-pipeline' plugin

expectations:
  * speed-up
  * resource preservation (don't needlessly run (full) builds when
    nothing pertinent changed)

caveat: this does not (just yet) work with Gitea (a rewrite underway)

[skip ci]

.drone.starlark

@@ -7,14 +7,39 @@ def main(ctx):
 			"kind": "pipeline",
 			"type": "docker",
 			"name": "golangci-lint",
+			"clone": {"disable": True},
 			"environment": {
 				"GOPROXY": "direct"
 			},
 			"steps": [
+				{
+					"name": "skip-check",
+					"image": "docker.io/immawanderer/drone-skip-pipeline:linux-amd64",
+					"pull": "always",
+					"settings": {
+						"rules": [
+							"*.go",
+							"*.sum",
+							"*.mod",
+							".golangci.yml"
+						]
+					},
+				},
+				{
+					"name": "clone",
+					"image": "docker.io/alpine/git",
+					"pull": "always",
+					"depends_on": ["skip-check"],
+					"commands": [
+						"git clone ${DRONE_GIT_HTTP_URL} .",
+						"git checkout ${DRONE_COMMIT}"
+					]
+				},
 				{
 					"name": "golangci-lint",
 					"image": "docker.io/immawanderer/archlinux-go-fyne:linux-amd64",
 					"pull": "always",
+					"depends_on": ["clone"],
 					"commands": [
 						"curl -sSfL " +
 						"https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh"+