fix pixi warning and version format for backend (#917)

Author: plandes

pyproject.toml

@@ -107,7 +107,7 @@ build-env = { features = ["pyverbase", "build-env"], solve-group = "default" }
 # specify where to download the build backend that builds conda files
 [tool.pixi.package.build.backend]
 name = "pixi-build-python"
-version = "2.0.0"
+version = "==2.0.0"
 channels = [
   "https://prefix.dev/pixi-build-backends",
   "https://prefix.dev/conda-forge",