feat: support alternative kubectl binaries

[mikecbone]

Proposed solution to #1514

There's not an easy & effective way, that I know of, to check for aliases within Go without running a bash shell in interactive mode and parsing the output. You then have to account for different shells, security risks etc

This seems like a straightforward way to support different kubectl binaries. It could alternatively be extracted into a flag.

For most users who have the kubectl binary as standard, this doesn't introduce any extra system calls