ci: fix version extraction, grep -oP unsupported on Alpine/BusyBox

The gitea-pages workflow runs in the alpine-jupyterbook container,
whose grep lacks PCRE (-P) support, so the version-extraction step
failed. Use a portable awk field scan instead, and fail loudly if
the version can't be found.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitea/workflows/deploy-pages.yml

@@ -22,7 +22,11 @@ jobs:
 
       - name: Extract project version from CMakeLists.txt
         run: |
-          VERSION=$(grep -oP 'project\(\s*musrfit\s+VERSION\s+\K[0-9]+\.[0-9]+\.[0-9]+' CMakeLists.txt)
+          VERSION=$(awk '/project\(musrfit/{for(i=1;i<=NF;i++) if($i=="VERSION"){print $(i+1); exit}}' CMakeLists.txt)
+          if [ -z "$VERSION" ]; then
+            echo "Could not extract musrfit version from CMakeLists.txt" >&2
+            exit 1
+          fi
           echo "MUSRFIT_VERSION=$VERSION" >> $GITHUB_ENV
 
       - name: Build musrfit-tech-docu