Fix Lektor installation of extension modules required by plugins.

The lektor-website/packages/project-data plugin requires
`readme_renderer[md]`, which, in turn requires `cmarkgfm<0.7.0`.
There is no wheel appropriate for the Windows GitHub runner
for `cmarkgfm==0.6.0`, so when Lektor goes to install the plugin,
the extension must be compiled.  Apparently the `ProgramData`
and `ProgramFiles` environment variables are important to this
process.

tox.ini

@@ -16,6 +16,11 @@ commands = pytest --cov={envsitepackagesdir}/lektor {posargs:tests}
 passenv =
     USERNAME
     PYTEST_ADDOPTS
+    # These are apparently necessary for Lektor to be able to install
+    # distributions requiring the compilation of extension modules on
+    # Windows.  (At least on the GitHub-hosted runner.)
+    ProgramData
+    ProgramFiles*
 setenv =
     # Use per-testenv coverage files to prevent contention when parallel
     # tests (using `tox -p`)