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.

website/sandbox/learnpianochords/shell.nix

@@ -1,5 +1,6 @@
 let
-  pkgs = import /home/wpcarro/nixpkgs {};
+  briefcase = import <briefcase> {};
+  pkgs = briefcase.third_party.pkgs;
 in pkgs.mkShell {
   buildInputs = with pkgs; [
     elmPackages.elm

website/sandbox/learnpianochords/src/server/default.nix

@@ -1,5 +1,5 @@
 let
-  briefcase = import /home/wpcarro/briefcase {};
+  briefcase = import <briefcase> {};
 in briefcase.buildHaskell.program {
   name = "server";
   srcs = builtins.path {

website/sandbox/learnpianochords/src/server/shell.nix

@@ -1,5 +1,5 @@
 let
-  briefcase = import /home/wpcarro/briefcase {};
+  briefcase = import <briefcase> {};
 in briefcase.buildHaskell.shell {
   deps = hpkgs: with hpkgs; [
     hspec