Automatically format ci pipeline specs with prettier (#13441)

* [ci] Deduplicate variables: sections in pipeline specs

The prettier yaml parser doesn't like these.

* [ci] provide git clean filter to format pipeline specs

* [ci] Reformat pipeline specs with prettier

substrate/scripts/ci/gitlab/prettier.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+
+# meant to be installed via
+#   git config filter.ci-prettier.clean "scripts/ci/gitlab/prettier.sh"
+
+prettier --parser yaml