Fix path handling in archive.py to always be posix and not native paths

[luna-duclos]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

No tests have been changed or added, but existing tests should validate the new behaviour still results in valid images

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

Currently, rules_docker when executed on windows will build a tar file with \ characters in the tar, this works fine when opened with tools like 7z, however, when extracting using a typical tar tool (or when docker does it), the files end up mangled with <filename>` all jumbled up together in their file name

This is because archive.py makes a lot of assumptions about posix paths using / characters, but then bundles this up with also using os.path instead of posixpath

What is the new behavior?

This fixes this problem by using posixpath in archive.py

Does this PR introduce a breaking change?

• Yes
• No

Other information

I realize windows support is spotty and partial at best, but this gets us going :)

[luna-duclos]

Would love some help figuring out whats failing on the mac, os.path should have already been posixpath on that platform. so its confusing me

[luna-duclos]

Aaand tests are green! Good to review :)

[gravypod]

Looks good. Thanks for doing this!