`exec.LookPath()` and `exec.Command()` aren't supposed to work with
binaries that are located in the current working directory:
fd208c8850:src/os/exec/exec.go
If `kubectl` is found in the current working directory, `LookPath()`
will return `ErrDot` even if it is also found in any other directory in
the `PATH`.
This patch detects this condition and shows a meaningful error message
in the log so that the user knows the reason why `kubectl` couldn't be
executed.
Related: https://github.com/derailed/k9s/issues/1787
2d2baa884a