Can't build with --no-isolation without installing the builder first

[mjpieters]

Minimal reproducable test case using this project itself:

# setup; temp dir with venv and build 0.10.0 source
mkdir /tmp/testnonisolated
cd /tmp/testnonisolated
git clone --depth 1 --branch 0.10.0 https://github.com/pypa/build.git
python -m venv venv
source venv/bin/activate
# install build with pip (isolated build)
pip install build/
# attempt to use build to build build (say that 10 times really fast)
python -m build --no-isolation build

This fails with

* Getting build dependencies for sdist...

Traceback (most recent call last):
  File "/private/tmp/testnonisolated/venv/lib/python3.11/site-packages/pyproject_hooks/_impl.py", line 321, in _call_hook
    raise BackendUnavailable(data.get('traceback', ''))
pyproject_hooks._impl.BackendUnavailable: Traceback (most recent call last):
  File "/private/tmp/testnonisolated/venv/lib/python3.11/site-packages/pyproject_hooks/_in_process/_in_process.py", line 77, in _build_backend
    obj = import_module(mod_path)
          ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/mj/.pyenv/versions/3.11.2/lib/python3.11/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen importlib._bootstrap>", line 1206, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1178, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1128, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "<frozen importlib._bootstrap>", line 1206, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1178, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1142, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'flit_core'

ERROR Backend 'flit_core.buildapi' is not available.

I can see that an attempt is made at installing the build dependencies, but this fails because ProjectBuilder.check_dependencies() calls ProjectBuilder.get_requires_for_build(), which relies on the builder to be installed already.

Either:

• install the builder first, separately, like an isolated install would do
• or, document that --no-isolation will only work if you manually install the builder first. This is what pip install --no-build-isolation does.

[layday]

[...] or, document that --no-isolation will only work if you manually install the builder first. This is what pip install --no-build-isolation does.

This is the intention. PR to improve the help text welcome.