$ env - python3.9 setup.py build
... (skipped many lines) ...
writing manifest file 'src/Pillow.egg-info/SOURCES.txt'
running build_ext
Traceback (most recent call last):
  File "$(BUILD_DIR)/setup.py", line 993, in <module>
    setup(
  File "/usr/lib/python3.9/vendor-packages/setuptools/__init__.py", line 108, in setup
    return distutils.core.setup(**attrs)
  File "/usr/lib/python3.9/vendor-packages/setuptools/_distutils/core.py", line 185, in setup
    return run_commands(dist)
  File "/usr/lib/python3.9/vendor-packages/setuptools/_distutils/core.py", line 201, in run_commands
    dist.run_commands()
  File "/usr/lib/python3.9/vendor-packages/setuptools/_distutils/dist.py", line 969, in run_commands
    self.run_command(cmd)
  File "/usr/lib/python3.9/vendor-packages/setuptools/dist.py", line 1213, in run_command
    super().run_command(command)
  File "/usr/lib/python3.9/vendor-packages/setuptools/_distutils/dist.py", line 988, in run_command
    cmd_obj.run()
  File "/usr/lib/python3.9/vendor-packages/setuptools/_distutils/command/build.py", line 132, in run
    self.run_command(cmd_name)
  File "/usr/lib/python3.9/vendor-packages/setuptools/_distutils/cmd.py", line 318, in run_command
    self.distribution.run_command(command)
  File "/usr/lib/python3.9/vendor-packages/setuptools/dist.py", line 1213, in run_command
    super().run_command(command)
  File "/usr/lib/python3.9/vendor-packages/setuptools/_distutils/dist.py", line 988, in run_command
    cmd_obj.run()
  File "/usr/lib/python3.9/vendor-packages/setuptools/command/build_ext.py", line 84, in run
    _build_ext.run(self)
  File "/usr/lib/python3.9/vendor-packages/Cython/Distutils/old_build_ext.py", line 186, in run
    _build_ext.build_ext.run(self)
  File "/usr/lib/python3.9/vendor-packages/setuptools/_distutils/command/build_ext.py", line 346, in run
    self.build_extensions()
  File "$(BUILD_DIR)/setup.py", line 438, in build_extensions
    if _cmd_exists(os.environ.get("PKG_CONFIG", "pkg-config")):
  File "$(BUILD_DIR)/setup.py", line 248, in _cmd_exists
    for path in os.environ["PATH"].split(os.pathsep)
  File "/usr/lib/python3.9/os.py", line 679, in __getitem__
    raise KeyError(key) from None
KeyError: 'PATH'
$