Run shellcheck on swiftly-install.sh

[patrickfreed]

shellcheck is a static analysis tool for bash which can catch a number of common bash issues. We should run it on swiftly-install.sh and include it in the soundness check CI task.

[adam-fowler]

This should be part of the CI setup as well

[Zingam]

bash is a kind of deprecated on macOS. Better use posix shell for max compatibility.

[adam-fowler]

bash is a kind of deprecated on macOS. Better use posix shell for max compatibility.

Do you mean deprecated in that it isn't the default shell on macOS anymore or that Apple intend to delete it in a future release of macOS. If it is the second can you post where Apple announced this.

Currently bash is available on pretty much every Linux release. I'll have to let @patrickfreed cover any issues with falling back to posix shell.