ci: cache and prefetch WebDrop dependencies

.woodpecker/release-backend.yml

@@ -32,7 +32,9 @@ steps:
       - release_tag="${CI_COMMIT_TAG:-${RELEASE_TAG:-}}"
       - test -n "$release_tag"
       - node scripts/check-api-contract.mjs "$release_tag"
-      - dotnet test server/WebDrop.slnx --configuration Release --verbosity minimal
+      - mkdir -p "$HOME/.cache/woodpecker/webdrop/nuget"
+      - NUGET_PACKAGES="$HOME/.cache/woodpecker/webdrop/nuget" dotnet restore server/WebDrop.slnx
+      - NUGET_PACKAGES="$HOME/.cache/woodpecker/webdrop/nuget" dotnet test server/WebDrop.slnx --configuration Release --verbosity minimal --no-restore
 
   publish-backend:
     image: bash