Don't parse and then reformat shasum hash lines

[edwbuck]

When generating sha256sum hashes of the release executables, we cut the output to obtain the hash and then join that back to the file name, in a way that ensures the output file name lacks any parent directory paths.

The problem is that this destroys the specific formatting of the hashing algorithm, such that on some platforms the line isn't recognized as valid input to the hashing algorithm's --check command line option.

Instead of cutting the output line, we need to change to the directory of the file to be hashed and then request the hashing of the filename only, lacking any parent paths. The output of that command should be written to the hash file without parsing.

All of this should be done in a sub-shell, avoiding the chance that a command leaves the current working directory in the wrong state should any of the internal commands to generate the hash or write it to a file fail.