Revise previous opinions about absolute paths GT <bracket-notation>

Unforeseen problem: `buildkite-agent` runs its builds in a separate directory,
so if I want the `nix-build` command to build the newly checked out code, I need
to set <briefcase> to the CWD.

ci/scripts/build-socrates.sh

@@ -1,6 +1,7 @@
 set -euo pipefail
 
 nix-build /home/wpcarro/nixpkgs/nixos \
-  -I nixos-config=/home/wpcarro/briefcase/nixos/socrates/default.nix \
+  -I briefcase="$(pwd)" \
+  -I nixos-config=nixos/socrates/default.nix \
   -A system \
   --show-trace