studio: make uv optional + fix --system for Colab

Three fixes based on review:

1. Make uv truly optional: _bootstrap_uv() now only checks if uv is
   already on PATH. It no longer tries to pip install uv. If uv is
   not present, pip is used with zero changes to behavior.

2. Add --system flag for Colab: on Colab there is no venv (packages
   install into system Python). uv requires --system in this case,
   otherwise it errors with "No virtual environment found". Added
   _in_virtualenv() check that detects VIRTUAL_ENV, sys.real_prefix,
   or sys.base_prefix != sys.prefix.

3. Fix label printed twice on uv fallback: when uv fails and falls
   back to pip, the label now says "(pip)" to distinguish from the
   initial uv attempt, instead of printing the same label twice.

Tested:
  - venv path: no --system flag, uv installs correctly
  - no-venv path (Colab sim): --system flag added automatically
  - full unsloth studio setup + training run (Llama-3.2-1B, 10 steps)

studio/install_python_stack.py

@@ -101,15 +101,16 @@ USE_UV = False  # Set by _bootstrap_uv() at the start of install_python_stack()
 
 
 def _bootstrap_uv() -> bool:
-    """Try to use uv for faster installs. Returns True if uv is available."""
-    if shutil.which("uv"):
-        return True
-    result = subprocess.run(
-        [sys.executable, "-m", "pip", "install", "--quiet", "uv"],
-        stdout = subprocess.PIPE,
-        stderr = subprocess.STDOUT,
+    """Check if uv is available on PATH. Does not install uv."""
+    return shutil.which("uv") is not None
+
+
+def _in_virtualenv() -> bool:
+    """Check if we are running inside a virtual environment."""
+    return os.environ.get("VIRTUAL_ENV") is not None or (
+        hasattr(sys, "real_prefix")
+        or (hasattr(sys, "base_prefix") and sys.base_prefix != sys.prefix)
     )
-    return result.returncode == 0 and shutil.which("uv") is not None
 
 
 def _filter_requirements(req: Path, skip: set[str]) -> Path:
@@ -154,6 +155,8 @@ def _build_pip_cmd(args: tuple[str, ...]) -> list[str]:
 def _build_uv_cmd(args: tuple[str, ...]) -> list[str]:
     """Build a uv pip install command with translated flags."""
     cmd = ["uv", "pip", "install"]
+    if not _in_virtualenv():
+        cmd.append("--system")
     cmd.extend(_translate_pip_args_for_uv(args))
     cmd.append("--torch-backend=auto")
     return cmd
@@ -193,7 +196,7 @@ def pip_install(
                 print(result.stdout.decode(errors = "replace"))
 
         pip_cmd = _build_pip_cmd(args) + constraint_args + req_args
-        run(label, pip_cmd)
+        run(f"{label} (pip)" if USE_UV else label, pip_cmd)
     finally:
         if actual_req is not None and actual_req != req:
             actual_req.unlink(missing_ok = True)