PIP_NO_BUILD_ISOLATION not set in build environment

[jjhelmus]

Despite the being specified in build.py, #3053, PIP_NO_BUILD_ISOLATION is not being set in the build/host environment.

package:
  name: foobar
  version: 1.0.0

build:
  script:
    - env | grep PIP_NO
    - exit 1

$ conda build --version
read filter "zstd" is not supported
write filter "zstd" is not supported
conda-build 3.17.4
$ conda build .
...
BUILD START: ['foobar-1.0.0-0.tar.bz2']
source tree in: /home/jhelmus/anaconda3/conda-bld/foobar_1544729597284/work
export PREFIX=/home/jhelmus/anaconda3/conda-bld/foobar_1544729597284/_h_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_pla
export SRC_DIR=/home/jhelmus/anaconda3/conda-bld/foobar_1544729597284/work
PIP_NO_INDEX=True
PIP_NO_DEPENDENCIES=True
Traceback (most recent call last):

[jjhelmus]

This causes issues building projects which have a pyproject.yaml file, for discussion see conda-forge/betse-feedstock#10

[jjhelmus]

Looks like the environment values are only written if they evaluate to truthy, so falsy values, including False are not written.

    with open(env_file, 'w') as bf:
        for k, v in env.items():
            if v:
                bf.write('export {0}="{1}"\n'.format(k, v))

[github-actions]

Hi there, thank you for your contribution!

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