Fix `not found` error at Codespaces

[xtetsuji]

Codespaces environment causes above error.

bash: is_darwin: command not found
bash: is_linux: command not found
bash: is_kde: command not found
bash: is_cygwin: command not found
bash: is_cursor_local: command not found
bash: is_cursor_codespaces: command not found
bash: exists: command not found
bash: exists: command not found
bash: exists: command not found
bash: exists: command not found
bash: exists: command not found
bash: exists: command not found