.envrc: Replace .envrc with .envrc.sample

The `.envrc` file acts as a personal setup file, and should not be checked in.

The team provides `.envrc.sample` where contributors can choose to:

    echo 'source_env .envrc.sample' >> .envrc

To bootstrap their own `.envrc` file.

.envrc

@@ -1,3 +0,0 @@
-watch_file rust-toolchain.toml
-
-use flake

.envrc.sample

@@ -0,0 +1,5 @@
+# Developers that want to use this can add 'source_env .envrc.sample' in their .envrc
+if has nix; then
+   watch_file rust-toolchain.toml
+   use flake
+fi

.gitignore

@@ -3,5 +3,6 @@
 /build/artifacts
 /build/*.tar.gz
 /.pre-commit-config.yaml
+/.envrc
 /.direnv/
 /result