Allow install to proceed even if there's no place to build wheels.

Fixes pypa#6158

Author: tgs

src/pip/_internal/wheel.py

@@ -840,13 +840,6 @@ def build(
             newly built wheel, in preparation for installation.
         :return: True if all the wheels built correctly.
         """
-        # TODO: This check fails if --no-cache-dir is set. And yet we
-        #       might be able to build into the ephemeral cache, surely?
-        building_is_possible = self._wheel_dir or (
-            autobuilding and self.wheel_cache.cache_dir
-        )
-        assert building_is_possible
-
         buildset = []
         format_control = self.finder.format_control
         for req in requirements: