- Use bash's [[ ]] instead of [ ] for conditional tests (S7688).
- Copy positional parameters into named locals in render(), has() and
sha256() (S7679).
- End the env.sh helper functions with an explicit return, which keeps
the last command's status (S7682).
- Give the appimagetool checksum case a default branch for architectures
without a pinned download (S131).
No behaviour change; shellcheck -x is clean.
Dylan M. Taylor