.drone.yml: use explizit version definition

.drone.yml

@@ -3,14 +3,14 @@ name: build
 
 steps:
 - name: gofmt
-  image: golang:1.21
+  image: golang:1.21.7
   commands:
     - gofmt -l -s .
   when:
     event:
       - push
 - name: vuln-check
-  image: golang:1.21
+  image: golang:1.21.7
   commands:
     - go install golang.org/x/vuln/cmd/govulncheck@latest
     - govulncheck ./...
@@ -111,4 +111,4 @@ volumes:
 when:
   event:
     exclude:
-      - pull_request
+      - pull_request