`SCIE=split` fails for scie-tote / files with a non-scie `"source"`.

[John Sirois]

For example, trying to split a scie-pants scie nets:

$ SCIE=split scie-pants /tmp/scie-pants-split
Error: Failed to open scie-tote zip: invalid Zip archive: Invalid zip header
$ ls -l /tmp/scie-pants-split/
total 7280
-rw-r--r-- 1 jsirois jsirois       0 Dec 19 12:27 cpython-3.8.15+20221106-aarch64-apple-darwin-install_only.tar.gz
-rw-r--r-- 1 jsirois jsirois 5780260 Dec 19 12:27 cpython-3.8.15+20221106-aarch64-unknown-linux-gnu-install_only.tar.gz
-rw-r--r-- 1 jsirois jsirois       0 Dec 19 12:27 cpython-3.8.15+20221106-x86_64-apple-darwin-install_only.tar.gz
-rw-r--r-- 1 jsirois jsirois       0 Dec 19 12:27 cpython-3.8.15+20221106-x86_64-unknown-linux-gnu-install_only.tar.gz
-rw-r--r-- 1 jsirois jsirois       0 Dec 19 12:27 cpython-3.9.15+20221106-aarch64-apple-darwin-install_only.tar.gz
-rw-r--r-- 1 jsirois jsirois       0 Dec 19 12:27 cpython-3.9.15+20221106-aarch64-unknown-linux-gnu-install_only.tar.gz
-rw-r--r-- 1 jsirois jsirois       0 Dec 19 12:27 cpython-3.9.15+20221106-x86_64-apple-darwin-install_only.tar.gz
-rw-r--r-- 1 jsirois jsirois       0 Dec 19 12:27 cpython-3.9.15+20221106-x86_64-unknown-linux-gnu-install_only.tar.gz
-rwxr-xr-x 1 jsirois jsirois 1668304 Dec 19 12:27 scie-jump