feat(tools): Add dispatcher script to transparently access tools

Initial version of tool provider via Nix. This requires two separate
steps for adding a new tool:

1. New symlink in tools/bin to point at the dispatch script.
2. Mapping of tool to Nix package set attribute in dispatch script.

.envrc

@@ -0,0 +1,4 @@
+# Configure the local PATH to contain tools which are fetched ad-hoc
+# from Nix.
+
+export PATH="${PWD}/tools/bin:${PATH}"