fix: update pip installation command in nixpacks.toml

Author: Devasy

nixpacks.toml

@@ -1,10 +1,10 @@
 [phases.setup]
-nixPkgs = ["python312", "pip"]
+nixPkgs = ["python312"]
 
 # Use the default Python install behavior but ensure we use the right requirements file
 [phases.install]
 cmds = [
-    "pip install --upgrade pip",
+    "python -m pip install --upgrade pip",
     "pip install -r requirements.txt"
 ]