explicit install nextflow

Author: Joon-Klaps

.github/workflows/build-docs.yml

@@ -5,6 +5,7 @@ on:
       - main
       - dev
       - master
+  workflow_dispatch:
 
 permissions:
   contents: write
@@ -37,6 +38,8 @@ jobs:
             mkdocs-material-
       - name: Install dependencies
         run: pip install mkdocs-material pymdown-extensions pillow cairosvg mike nf-core
+      - name: Install Nextflow
+        uses: nf-core/setup-nextflow@v2
       - name: Build parameter docs
         run: nf-core schema docs --format markdown --columns parameter,description,default --output docs/parameters.md --force
       - name: Read parameter tip