Use coherent env. in release.yml (#4075)

Makes release builds aware of the ghc-version to build with. (and also
stack and hpack, which are minor)

Author: jberthold

.github/workflows/release.yml

@@ -4,6 +4,11 @@ on:
     branches:
       - release
 
+env:
+  ghc_version: "9.6.5"
+  stack_version: "2.15.1"
+  hpack_version: '0.36'
+
 jobs:
   draft-release:
     name: 'Draft Release'