When the YOSYS environment variable isn't defined, the fallback yosys is used as the name of the yosys executable (src). When this fails (i.e. the environment variable is not defined and yosys is not in the user's PATH), a file not found error is thrown.

Suggestion: add a more user-friendly error message that prompts the user to define/check the YOSYS environment variable or add yosys to their PATH