Use macOS-compliant shell command for generating build timestamp

The previous command only worked on Linux.

Author: per1234

Taskfile.yml

@@ -185,7 +185,7 @@ vars:
   COMMIT:
     sh: echo "$(git log -n 1 --format=%h)"
   TIMESTAMP:
-    sh: echo "$(date --utc --iso-8601=second)"
+    sh: echo "$(date -u +"%Y-%m-%dT%H:%M:%SZ")"
   LDFLAGS: >
     -ldflags '-X github.com/arduino/arduino-check/configuration.commit={{.COMMIT}} -X github.com/arduino/arduino-check/configuration.buildTimestamp={{.TIMESTAMP}}'
   GOFLAGS: "-timeout 10m -v -coverpkg=./... -covermode=atomic"