Hardcode using bash as make's shell

On Debian/Ubuntu the default shell is dash, and builds are done with SHELL overridden to dash. Since archlinux-keyring now has Bash-isms in the install and uninstall targets (for {} expansion), and rewriting it to drop this is inconvenient (because we'll have to repeat the path prefixes), hardcode the use of Bash instead.

Use /bin/bash to be compatible with distros that have not finished the /bin -> /usr/bin migration yet.

Signed-off-by: Michel Alexandre Salim michel@michel-slm.name