Give each bootstrap phase its own subdirectory

tools/bootstrap.py

 HERE = Path(__file__).parent.absolute()
 PROJECT_ROOT = HERE.parent
 BUILD_DIR = PROJECT_ROOT / '_build'
-BOOTSTRAP_DIR = BUILD_DIR / '_bootstrap'
+BOOTSTRAP_BASE_DIR = BUILD_DIR / '_bootstrap'
 PREBUILT_DIR = PROJECT_ROOT / '_prebuilt'
 
 EXE_SUFFIX = '.exe' if os.name == 'nt' else ''
         raise subprocess.CalledProcessError(res.returncode, cmd)
 
 
-def _clone_bootstrap_phase(ph: str) -> None:
+def _clone_bootstrap_phase(ph: str) -> Path:
     print(f'Cloning: {ph}')
-    if BOOTSTRAP_DIR.exists():
-        shutil.rmtree(BOOTSTRAP_DIR)
+    bts_dir = BOOTSTRAP_BASE_DIR / ph
+    if bts_dir.exists():
+        shutil.rmtree(bts_dir)
     _run_quiet([
         'git',
         'clone',
         '--depth=1',
         f'--branch={ph}',
         f'file://{PROJECT_ROOT}',
-        BOOTSTRAP_DIR,
+        bts_dir,
     ])
+    return bts_dir
 
 
-def _build_bootstrap_phase(ph: str, args: argparse.Namespace) -> None:
+def _build_bootstrap_phase(ph: str, bts_dir: Path, args: argparse.Namespace) -> None:
     print(f'Running build: {ph} (Please wait a moment...)')
     env = os.environ.copy()
     env['DDS_BOOTSTRAP_PREV_EXE'] = str(PREBUILT_DIR / 'dds')
         [
             sys.executable,
             '-u',
-            str(BOOTSTRAP_DIR / 'tools/build.py'),
+            str(bts_dir / 'tools/build.py'),
             f'--cxx={args.cxx}',
         ],
         env=env,
     )
 
 
-def _pull_executable() -> Path:
+def _pull_executable(bts_dir: Path) -> Path:
     prebuild_dir = (PROJECT_ROOT / '_prebuilt')
     prebuild_dir.mkdir(exist_ok=True)
-    generated = list(BOOTSTRAP_DIR.glob(f'_build/dds{EXE_SUFFIX}'))
+    generated = list(bts_dir.glob(f'_build/dds{EXE_SUFFIX}'))
     assert len(generated) == 1, repr(generated)
     exe, = generated
     dest = prebuild_dir / exe.name
 
 
 def _run_boot_phase(phase: str, args: argparse.Namespace) -> Path:
-    _clone_bootstrap_phase(phase)
-    _build_bootstrap_phase(phase, args)
-    return _pull_executable()
+    bts_dir = _clone_bootstrap_phase(phase)
+    _build_bootstrap_phase(phase, bts_dir, args)
+    return _pull_executable(bts_dir)
 
 
 def main(argv: Sequence[str]) -> int: