docs[patch]: Improve env variable handling in documentation notebooks (#26347)

Updated using gritql

```
`os.environ[$key] = getpass($msg)` as $M where {
    $M <: ! within if_statement(),
    $M => `if $key not in os.environ: $M`
}
```

cookbook/code-analysis-deeplake.ipynb

@@ -90,7 +90,8 @@
     "import os\n",
     "from getpass import getpass\n",
     "\n",
-    "os.environ[\"OPENAI_API_KEY\"] = getpass()\n",
+    "if \"OPENAI_API_KEY\" not in os.environ:\n",
+    "    os.environ[\"OPENAI_API_KEY\"] = getpass()\n",
     "# Please manually enter OpenAI Key"
    ]
   },