Fixed bench workflow + changeprefix issue in prefix releases

changeprefix.py only works on prefixes, which is a bit of a problem for
flags in the workflow scripts, requiring extra handling to not hide the prefix
from changeprefix.py

.github/workflows/test.yml

@@ -404,7 +404,7 @@ jobs:
           valgrind --version
       - name: bench
         run: |
-          make bench BENCHFLAGS+=-olfs.bench.csv
+          make bench BENCHFLAGS+="-o lfs.bench.csv"
 
           # find bench results
           ./scripts/summary.py lfs.bench.csv \