ci: fix missing "current version" in commit message from `bump_versio…

…n` action

Signed-off-by: Daniel Weiße <[email protected]>

.github/actions/bump_version/action.yml

@@ -27,6 +27,7 @@ runs:
       run: |
         cd ${{ inputs.working-directory }}
 
+        current=$(<version.txt)
         echo -n "${{ inputs.version }}" > version.txt
 
         commit_msg="flake: ${current} -> ${{ inputs.version }}"