(linux) PineFlash panics without a friendly message if `pkexec` cannot be found

[acuteaangle]

If pkexec is missing or cannot be found, PineFlash will panic with the following user-unfriendly error when it tries to prompt for superuser privileges:

thread 'main' panicked at src/submodules/flash.rs:196:18:
Could not flash soldering iron: Os { code: 2, kind: NotFound, message: "No such file or directory" }

Expected: PineFlash would display a message that helps identify the more specific cause of the error:

Fatal: could not run pkexec; encountered "No such file or directory" while attempting to execute pkexec dfu-util -D /tmp/v2.22-Pinecilv2 -a 0; Is polkit installed?