Support build-briefcase.sh

For now, I'm supporting two CI pipelines:
- build-socrates
- build-briefcase

Conceptually, build-briefcase should cover what build-socrates does now, but
eventually I would like build-socrates to call `switch-to-configuration` so that
all of my websites, etc. stay fresh.

ci/scripts/build-briefcase.sh

@@ -0,0 +1,6 @@
+set -euo pipefail
+
+nix-build . \
+  -I briefcase="$(pwd)" \
+  -I nixpkgs=/home/wpcarro/nixpkgs-channel \
+  --show-trace