fix: move starship init bash to /etc/profile.d (#110)

We are doing shell detection this way because people will do weird stuff
with their $0 or $SHELL to make this more robust.

Related: ublue-os/aurora#1519

fixes: ublue-os/aurora#1518

Author: renner0e

system_files/shared/etc/profile.d/90-aurora-starship.sh

@@ -0,0 +1,6 @@
+# shellcheck shell=sh
+command -v starship >/dev/null 2>&1 || return 0
+
+if [ "$(basename "$(readlink /proc/$$/exe)")" = "bash" ]; then
+  eval "$(starship init bash)"
+fi