nuget updater command is already space-enabled; allow unsafe execution

[brettfo]

The NuGet updater is invoked with already-escaped command line options, so we can simply call it with allow_unsafe_shell_command: true, otherwise the arguments are double escaped.

Adding that parameter was the real fix, but I also refactored the code a bit to make testing the command line generation easier.

N.b., the helper function run_shell_command only takes a string which it tries to escape and that's where the problem arose; we have to pre-escape our strings to allow for a path with spaces in it, etc., and then tell run_shell_command that we've already escaped everything.

Fixes #8633

[JamieMagee]

Approved with some small comments.