(#2379) Correct pack files filter for Linux

[TheCakeIsNaOH]

If there is not a element in the .nuspec, then the pack command
will gather up a list of all of files the in the directory which is
used to include them in the package. However, the nuspec should not be
included in this list, because it is auto-regenerated. The .nupkg also
should not be included. Therefore, there is a filter to remove the
.nuspec and .nupkg from the list of files.

However, this filter has the directory separator hardcoded as a
backslash. This does not work correctly on platforms where the
directory separator is a forward slash. Therefore, this fixes
the directory separator to be correct depending on what platform choco
is running on.

This patch was written by @AdmiringWorm, I just created the reproduction and packaged it up into a commit.
https://github.com/AdmiringWorm/chocolatey-arch/blob/master/0003-Use-correct-exclude-filer-on-unix.patch

Fixes #2379

I thought that the partUri issue was fixed by #2276, but it seems like there are actually two different problems that caused it.

[gep13]

LGTM!

[gep13]

This was tested on my Twitch stream this evening, we did the following:

1. Used this folder structure as a test package: https://github.com/Starz0r/ChocolateyPackagingScripts/tree/master/src/templates/dust
2. NOTE: Had to remove the description element in the nuspec as this caused a different problem
3. Ran the docker image for Chocolatey, and attmepted choco pack and got the error indicated
4. Added files element to nuspec and re-ran choco pack everything worked as expected
5. Checked out PR branch and re-built docker image using new code
6. Ran choco pack without files element and it worked as expected
7. Ran choco pack with files element and it also worked as expected

[gep13]

@TheCakeIsNaOH thank you very much for getting to the bottom of this problem, really appreciate it!