ci: Moved doc setup to separate script (#3339)

Since there seems to be some problem with the doc setup on the GitHub
runners, let's try running it as a separate script.

melos.yaml

@@ -54,14 +54,7 @@ scripts:
     description: Run dartdoc checks for all packages.
 
   doc-setup:
-    steps:
-      - 'echo Checking python version:'
-      - 'python3 --version && python3 -c "import sys; sys.exit(0 if sys.version_info >= (3,8) else 2)" || (echo "Error: Python 3.8+ is required" && exit 1)'
-      - 'echo Installing required python modules:'
-      - 'python3 -m pip install -r "$MELOS_ROOT_PATH/doc/_sphinx/requirements.txt"'
-      - 'echo Installing dartdoc_json:'
-      - 'dart pub global activate dartdoc_json'
-      - 'echo Done.'
+    run: ./scripts/doc-setup.sh
     description: Prepares the environment for documentation development.
 
   doc-build:

scripts/doc-setup.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+set -e
+
+echo Checking python version:
+python3 --version && python3 -c "import sys; sys.exit(0 if sys.version_info >= (3,8) else 2)" || (echo "Error: Python 3.8+ is required" && exit 1)
+echo Installing required python modules:
+python3 -m pip install -r "$MELOS_ROOT_PATH/doc/_sphinx/requirements.txt"
+echo Installing dartdoc_json:
+dart pub global activate dartdoc_json
+echo Done.