Write version _number_ to CITATION.cff

While the version is generally represented as e.g. v1.4.0.post1,
in CITATION.cff the usual style is to omit the "v" and our existing
release.py script assumes this.

Solve the problem by tweaking the version-number-writing action.

.github/workflows/set_version.yml

@@ -23,11 +23,13 @@ jobs:
           import os
           from packaging.version import Version
 
-          version_string = 'v' + str(Version("${{ inputs.version }}"))
+          version_number = str(Version("${{ inputs.version }}"))
+          version_string = 'v' + version_number
 
           print(f"Normalised version string: {version_string}")
 
           with open(os.environ["GITHUB_ENV"], "a") as github_env:
+              print(f"VERSION_NUMBER={version_number}", file=github_env)
               print(f"VERSION_STRING={version_string}", file=github_env)
 
       - uses: actions/checkout@v4
@@ -45,7 +47,7 @@ jobs:
       - name: "Update CITATION.cff"
         shell: bash -l {0}
         run: |
-          sed -i "s/^version:\ .*/version: ${VERSION_STRING}/" CITATION.cff
+          sed -i "s/^version:\ .*/version: ${VERSION_NUMBER}/" CITATION.cff
           sed -i "s/^date-released:\ .*/date-released: $(date -I)/" CITATION.cff
 
       - name: "Create commit"