chmod post-action broken by recent .Net Core changes

[rmunn]

See https://github.com/dotnet/corefx/issues/24704#issuecomment-393750554 (and the whole issue discussion above that comment) for details, but basically, the meaning of UseShellExecute has changed. It no longer means "use /bin/sh"; now it means "use xdg-open". This results in errors like the following when the chmod post-action is used:

Processing post-creation actions...
xdg-open: unexpected argument '+x'
Try 'xdg-open --help' for more information.
Unable to apply permissions +x to "*.sh".
Post action failed.
Description: Make scripts executable
Manual instructions: Run 'chmod +x *.sh'

The only fix I can think of so far is to change the #1042 logic to run /bin/sh with arguments $"-c \"chmod {entry.Key} {file}\"" (note that the quotation marks after the -c MUST be double-quotation marks so that globs like *.sh will be expanded by the shell; single-quotes mean "Do not expand globs" to the Unix shell, which would not be the desired behavior here),

Also see #1028 (comment) and comments further down for even more details.