fix: keep is_exist unchanged in rfcs #5

Job	Run time
build-cpp-doc	25s
build-c-doc	27s
build-dav-server-opendalfs-doc	50s
build-fuse3-opendal-doc	1m 13s
build-lua-doc	9s
build-java-doc	16s
build-haskell-doc	3m 10s
build-nodejs-doc	21s
build-object-store-opendal-doc	40s
build-ocaml-doc	2m 15s
build-opendal-compat-doc	45s
build-parquet-opendal-doc	42s
build-python-doc	2m 51s
build-rust-doc	3m 58s
build-unftp-sbe-opendal-doc	1m 21s
build-virtiofs-opendal-doc	42s
build-website	44s
	20m 49s

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