Versioneer does not work in the Guix build container because:

* VCS information is unavailable
* the build directory does not have the supported "$name-$version" format
* as of 0.21, versioneer has no way to override the discovered values

This patch adds support for extracting version from the
GUIX_VERSIONEER_VERSION environment variable set by the set-version
phase of the pyproject build system.

--- a/src/get_versions.py
+++ b/src/get_versions.py
@@ -19,6 +19,10 @@
         # see the discussion in cmdclass.py:get_cmdclass()
         del sys.modules["versioneer"]
 
+    ver = {"version": os.getenv("GUIX_VERSIONEER_VERSION")}
+    if ver["version"] is not None:
+        return ver
+
     root = get_root()
     cfg = get_config_from_root(root)