Add BuildEnvironment._temp_dir to tempdir registry

Similar to the InstallRequirement temp build dir, now we'll be able to
refactor this to be globally managed.

src/pip/_internal/build_env.py

@@ -17,7 +17,7 @@ from pip._vendor.pkg_resources import Requirement, VersionConflict, WorkingSet
 
 from pip import __file__ as pip_location
 from pip._internal.utils.subprocess import call_subprocess
-from pip._internal.utils.temp_dir import TempDirectory
+from pip._internal.utils.temp_dir import TempDirectory, tempdir_kinds
 from pip._internal.utils.typing import MYPY_CHECK_RUNNING
 from pip._internal.utils.ui import open_spinner
 
@@ -54,7 +54,7 @@ class BuildEnvironment(object):
 
     def __init__(self):
         # type: () -> None
-        self._temp_dir = TempDirectory(kind="build-env")
+        self._temp_dir = TempDirectory(kind=tempdir_kinds.BUILD_ENV)
 
         self._prefixes = OrderedDict((
             (name, _Prefix(os.path.join(self._temp_dir.path, name)))

src/pip/_internal/cli/req_command.py

@@ -156,7 +156,7 @@ class IndexGroupCommand(Command, SessionCommandMixin):
             pip_self_version_check(session, options)
 
 
-KEEPABLE_TEMPDIR_TYPES = [tempdir_kinds.REQ_BUILD]
+KEEPABLE_TEMPDIR_TYPES = [tempdir_kinds.BUILD_ENV, tempdir_kinds.REQ_BUILD]
 
 
 def with_cleanup(func):

src/pip/_internal/utils/temp_dir.py

@@ -24,7 +24,8 @@ logger = logging.getLogger(__name__)
 # Kinds of temporary directories. Only needed for ones that are
 # globally-managed.
 tempdir_kinds = enum(
-    REQ_BUILD="req-build"
+    BUILD_ENV="build-env",
+    REQ_BUILD="req-build",
 )