Build should not use global target directory when running from install

If the user's pip.conf includes a target directory setting,
attempting to install a package in editable mode or from source
results in a fatal error during the installation of setuptools.

The following assertion triggers:
assert not (home and prefix), "home={} prefix={}".format(home, prefix)

To avoid this issue when building a package, the target
setting should be ignored. This can be achieved by passing an empty
target when installing dependencies in the BuildEnvironment class.

Author: Anton Zelenov

news/8438.bugfix.rst

@@ -0,0 +1,10 @@
+If the user's pip.conf includes a target directory setting,
+attempting to install a package in editable mode or from source
+results in a fatal error during the installation of setuptools.
+
+The following assertion triggers:
+assert not (home and prefix), "home={} prefix={}".format(home, prefix)
+
+To avoid this issue when building a package, the target
+setting should be ignored. This can be achieved by passing an empty
+target when installing dependencies in the BuildEnvironment class.

src/pip/_internal/build_env.py

@@ -242,6 +242,10 @@ def _install_requirements(
             prefix.path,
             "--no-warn-script-location",
             "--disable-pip-version-check",
+            # The prefix specified two lines above, thus
+            # target from config file or env var should be ignored
+            "--target",
+            "",
         ]
         if logger.getEffectiveLevel() <= logging.DEBUG:
             args.append("-vv")