Fix pip installation to use python -m pip

Author: Surya2Developer

nixpacks.toml

@@ -4,10 +4,11 @@ nixPkgs = ["python312Full"]
 # Use the default Python install behavior but ensure we use the right requirements file
 [phases.install]
 cmds = [
-    "pip install --upgrade pip",
-    "pip install -r requirements.txt"
+    "python -m pip install --upgrade pip",
+    "python -m pip install -r backend/requirements.txt"
 ]
 
+
 [phases.build]
 cmds = [
     "echo 'Build phase: Copying backend files'",