feat(devenv): allow disabling devenv locally with .envrc.user

Current devenv config has problems on at least Ubuntu 22.04.

.envrc.user with the below content enables work without devenv:

layout_poetry() {
 # create venv if it doesn't exist
 poetry run true

 export VIRTUAL_ENV=$(poetry env info --path)
 export POETRY_ACTIVE=1
 PATH_add "$VIRTUAL_ENV/bin"
}

layout_poetry
export SKIP=nixpkgs-fmt
export UNBLOB_USE_DEVENV=false

Author: e3krisztian

.envrc

@@ -1,4 +1,10 @@
-watch_file flake.nix flake.lock devenv.nix
-use nix \
-    --option extra-substituters "https://unblob.cachix.org" \
-    --option extra-trusted-public-keys "unblob.cachix.org-1:5kWA6DwOg176rSqU8TOTBXWxsDB4LoCMfGfTgL5qCAE="
+# Source further custom features from .env.user if exists
+# Also allows users to disable/override features in this file
+source_env_if_exists .envrc.user
+
+if ${UNBLOB_USE_DEVENV:-true}; then
+    watch_file flake.nix flake.lock devenv.nix
+    use nix \
+        --option extra-substituters "https://unblob.cachix.org" \
+        --option extra-trusted-public-keys "unblob.cachix.org-1:5kWA6DwOg176rSqU8TOTBXWxsDB4LoCMfGfTgL5qCAE="
+fi