systemd: fix build platform shebang reference
systemd: fix build platform shebang reference

patchShebangs was writing a build platform bash shebang to
systemd-update-helper, which ends up in the output. To fix this, this patch
restricts patchShebangs to only run on certain directories.

Also, remove a comment stating that patchShebangs will no longer be necessary
after the next systemd release. This is not the case because /usr/bin/env
doesn't exist within the sandbox and will still need to be patched.

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