This prevents starting a new shell with the IDF environment like this:
env IDF_PATH=/my/idf/path fish -C 'source "$IDF_PATH"/export.fish'
as `status current-command` returns `fish` in this case, but this check expects
`source`.
This check does nothing anyway, as you can't even execute that file even if it
was marked executable, as fish rejects files without a shebang, and it's not
valid bash.
Closes https://github.com/espressif/esp-idf/pull/5860
Roland Dobai