build: an env var to allow no-network tox runs

(for airplanes)

Author: nedbat

tox.ini

@@ -30,13 +30,15 @@ setenv =
     # so override any local setting.
     PYTHONPYCACHEPREFIX=
 
+# $set_env.py: COVERAGE_PIP_ARGS - Extra arguments for `pip install`
+# `--no-build-isolation` will let tox work with no network.
 commands =
     # Create tests/zipmods.zip
     python igor.py zip_mods
 
     # Build the C extension and test with the CTracer
     python setup.py --quiet build_ext --inplace
-    python -m pip install -q -e .
+    python -m pip install {env:COVERAGE_PIP_ARGS} -q -e .
     python igor.py test_with_tracer c {posargs}
 
     # Remove the C extension so that we can test the PyTracer
@@ -84,7 +86,7 @@ commands =
     python -m pylint --notes= {env:LINTABLE}
     check-manifest --ignore 'doc/sample_html/*,.treerc'
     # If 'build -q' becomes a thing (https://github.com/pypa/build/issues/188),
-    # this can be simplifed:
+    # this can be simplified:
     python igor.py quietly "python -m build"
     twine check dist/*