.github/workflows/release.yml: fix extraeneous double quote

Author: rouault

.github/workflows/release.yml

@@ -50,7 +50,7 @@ jobs:
           else
             export PRE_RELEASE="true"
           fi
-          echo "PRE_RELEASE"=$PRE_RELEASE" >> $GITHUB_OUTPUT
+          echo "PRE_RELEASE=$PRE_RELEASE" >> $GITHUB_OUTPUT
   mkgdaldist:
     runs-on: ubuntu-latest
     needs: safety_checks