Allow pkexec outside of /usr/bin

[jtojnar]

Problem description

Sublime Text hardcodes /usr/bin/pkexec, which does not exist on NixOS, so we have to replace it with /run/wrappers/bin/pkexec using a LD_PRELOAD hack. But since it is an environment variable, it gets passed to child programs, not all tolerating such interference.

(On NixOS, there is not really /usr directory. We symlink directory trees of globally installed packages into /run/current-system/sw, and /run/current-system/sw/bin directory is added to PATH environment variable, but packages cannot contain setuid binaries for security reasons – unprivileged users can build and install packages. Those are placed in /run/wrappers/bin, which is also added to PATH.)

Preferred solution

It would be nice if pkexec could be picked up from PATH.

Alternatives

• For software that has source code available, we just patch it but that is not an option here.
• The path in the source code could be padded from right with \0 so that we could patch the executable with a longer path. For us, nine extra characters should be enough but perhaps add few more to be safe.
• Define the path in Default.sublime-package. Since it would be defined in Python code, we can patch it easily.

[jtojnar]

Looks like run_admin_process now calls file_exists so replacing /usr/bin/pkexec for pkexec\x00\x00\x00\x00\x00\x00\x00\x00\x00 as we were doing and relying on PATH no longer works.