ci: Fix release-plz field config (#1117)

I set an invalid flag for release-plz...
https://release-plz.ieni.dev/docs/config#the-git_release_enable-field

This triggered an error in CI:
https://github.com/CQCL/hugr/actions/runs/9270279819/job/25502921479#step:4:216

release-plz.toml

@@ -25,12 +25,12 @@ release = true
 
 # Disabled until the first version is manually published
 publish = false
-github_release = false
+git_release_enable = false
 
 [[package]]
 name = "hugr-passes"
 release = true
 
 # Disabled until the first version is manually published
 publish = false
-github_release = false
+git_release_enable = false