Clear $NIX_PATH

Ensure that $NIX_PATH is empty using a .envrc file. nix.dev considers references
to <nixpkgs>, <briefcase>, <depot>, <unstable> to be an anti-pattern because it
hinders reproducibility by introducing an implicit environment.

This repository is chock-full of references to
<{nixpkgs,unstable,briefcase,depot}>, so right now everything is broken.

.envrc

@@ -1,6 +1,10 @@
 export BRIEFCASE="$(realpath ~/briefcase)"
 export DEPOT="$(realpath ~/depot)"
 export NIXPKGS="$(realpath ~/nixpkgs)"
-export NIX_PATH="nixpkgs=${NIXPKGS}:unstable=$(realpath ~/nixpkgs-channels):depot=${DEPOT}:briefcase=${BRIEFCASE}";
+# I'm ensuring that $NIX_PATH is unset to avoid the anti-pattern of depending on
+# <nixpkgs>, <briefcase>, <depot>, <unstable>.
+# For more information about setting this to "", see:
+# https://nix.dev/tutorials/towards-reproducibility-pinning-nixpkgs.html#pinning-nixpkgs
+export NIX_PATH="";
 export DESKTOP="zeno.lon.corp.google.com";
 export LAPTOP="seneca";