Fix tag reference in release action (#4)

Author: mdegat01

.github/workflows/release.yaml

@@ -55,7 +55,7 @@ jobs:
           with open("pyproject.toml", "rb") as f:
             pyproject = tomli.load(f)
 
-          pyproject["project"]["version"] = "${tag}"
+          pyproject["project"]["version"] = "${{ env.tag }}"
 
           with open("pyproject.toml", "wb") as f:
             tomli_w.dump(pyproject, f)