Deployment cache should include requirements.txt containing package versions

[schloerke]

I believe there needs to be a step where we install the app packages.

Then we should also include the installed packages in the cache path.

          path: |
            ${{ matrix.folder }}
            INSTALL_PATH_HERE

IDK what the Install location is for the python packages

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Motivation: We cache both the shiny code and the app content

Originally posted by @schloerke in #8 (comment)

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Oh that's a good point, what about including the shiny version in the requirements.txt file instead? I'll add an issue.

• @gshotwell