optionally use nom to build devshell dependencies

This is nice when messing around with the nix configuration, since
you'll often end up building rocksdb *many* times.

We shouldn't need to do anything to get the logs in CI because we're
calling `nix-build-and-cache ci` at the beginning

.envrc

@@ -2,6 +2,16 @@
 
 dotenv_if_exists
 
-use flake ".#${DIRENV_DEVSHELL:-default}"
+system="$(nix eval --impure --raw --expr 'builtins.currentSystem')"
+devshell="${DIRENV_DEVSHELL:-default}"
+
+if command -v nom &> /dev/null && [ -t 0 ]; then
+    # if nom is available, build the devshell dependencies with it to get nicer
+    # progress monitoring. Don't do this when stdout is piped, because it shows
+    # up weird in engage.
+    nom build ".#devShells.$system.$devshell"
+fi
+
+use flake ".#$devshell"
 
 PATH_add bin