Packaging instructions (for workstation) need updating

[rocodes]

The build wheels scripts need tweaking (post monorepo and just post a few different building changes).

• scripts/update-requirements has incorrect usage string (usage is PKG_DIR=/path/to/pkg make requirements, not the --pkg-dir option)
• readme should indicate that the path is PKG_DIR=/whatever/securedrop-client/subcomponent make requirements (/client, /export, etc)
• make build-wheels make target suggested by the make requirements script ("Please build the wheel by using the following command: PKG_DIR=/home/user/securedrop-client/export make build-wheels") errors out since the shasums file isn't located at /home/user/securedrop-builder/export/sha256sums.txt.asc)
• running through the wheel building steps manually: (./scripts/verify-sha256sum-signature securedrop-export/, PKG_DIR=../securedrop-client/export ./scripts/build-sync-wheels, ./scripts/sync-sha256sums securedrop-export/) works fine
• the suggested gpg command ("Now you must sign the generated sha256sums.txt file: gpg --armor --output securedrop-export/sha256sums.txt.asc --detach-sig securedrop-export/sha256sums.txt") does not work with Qubes split-gpg. For qubes split gpg users the command is qubes-gpg-client --armor --detach-sign $targetfile > $outputfile

Just confirming this looks right to folks before we make any changes

[rocodes]

I wasn't aware that the method I was using has been somewhat deprecated in favour of build-debs.sh in the client repo! Oops. In any case, whatever the preferred method for building components should be documented in this readme as well as the developer docs. :)