treewide: add 'shellcheck shell' directives

I used 'bash' for zsh files, since that's the closest and `shell=zsh` is
not supported [1].

[1]: https://github.com/koalaman/shellcheck/issues/809

bootstrap.sh

@@ -1,5 +1,6 @@
 #!/usr/bin/env nix-shell
 #! nix-shell -i bash -p bitwarden-cli git gnupg jq nix
+# shellcheck shell=bash
 
 # Command failure is script failure
 set -e