as_cmdstan_fit fails for paths using tilde expansion

[ksvanhorn]

Describe the bug
If a CSV file path starts with "~", indicating tilde expansion, cmdstanr::as_cmdstan_fit() gives an error saying the file does not exist, and (in contradiction of the previous) that the CSV file is corrupted.

To Reproduce
cmdstanr::as_cmdstan_fit(file_paths) where file_paths contains any path starting with "~/".

Expected behavior
I expect the normal tilde expansion to happen.

Operating system
macOS Ventura 13.4

CmdStanR version number
0.5.3

Additional context
Stepping through this in the debugger, the problem occurs within the function read_csv_metadata(), which calls data.table::fread(cmd = fread_cmd, ...), where fread_cmd was defined as

fread_cmd <- paste0(grep_path_quotes, " \"^[#a-zA-Z]\" --color=never \"", 
            csv_file, "\"")

The quotes placed around the file path in csv_file inhibit tilde expansion when the shell executes the command.