fix: keep a test's installer run off the WSL side

test_install loads its own install module. Its free-space test ran
main() in process without naming the OS, so on a WSL host main()
entered WSL mode and left _WSL active in that module, for the rest of
the process. Any test of the module run after it, as when the module
is named twice on one command line, read D:\bios as /mnt/d/bios. The
test now runs as Linux and restores _WSL on exit.

tests/test_install.py

@@ -428,7 +428,11 @@ class TheDataTreesCountInTheFreeSpace(unittest.TestCase):
 
         member = os.urandom(4096)
         results = []
-        with FakeRepository() as repo, unittest.mock.patch.dict(os.environ):
+        # The run is a Linux one whatever the host: under WSL main() reads
+        # the Windows side and keeps that in the module, where the Windows
+        # detection tests run after it found D:\ turned into /mnt/d.
+        with FakeRepository() as repo, unittest.mock.patch.dict(os.environ, RETROBIOS_OS="linux"), \
+                unittest.mock.patch.dict(install._WSL):
             listed = json.loads((repo.root / "install" / "retroarch.json").read_text())
             listed["data_directories"] = [{
                 "ref": "tree", "dest": "tree", "type": "zip", "archive_size": 50_000_000,