backup: replace FilesystemSink with fileblob

Our FilesystemSink has some subtle differences in behavior compared to the gocloud implementations. For instance, creating a new file and then canceling the context will leave an empty file, vs no change for gocloud.

Replace FilesystemSink with the fileblob sink for local filesystem operations. This does require us to adjust bundle tests to expect shared directory and file permissions, rather than private, as fileblob does not make this configurable.

We set the NoTempDir option as otherwise we would require /tmp and the target directory be on the same filesystem. Many customers will be saving backups to a mounted volume, and /tmp is commonly configured as a RAMdisk.

Closes #5879 (closed)