envrc: improve

- add shellcheck hint
- load .envrc.local if one exists
- use a variable to allow choosing the shell variant

Change-Id: Iea34e5a800f5d463e5792020c5c293b8b3071ca5

.envrc

@@ -1 +1,3 @@
-use flake
+# shellcheck shell=bash
+source_env_if_exists .envrc.local
+use flake ".#${LIX_SHELL_VARIANT:-default}" "${LIX_SHELL_EXTRA_ARGS[@]}"

.gitignore

@@ -147,6 +147,7 @@ result-*
 
 .vscode/
 .direnv/
+.envrc.local
 
 # clangd and possibly more
 .cache/