Optionally import justfile.local for developers

`just` is a useful tool for managing shell commands. Sometimes, there's
a just command that's useful just temporarily, in a given task, but
won't be generally useful, and shouldn't pollute the primary justfile.

So, we now attempt to import `justfile.local` if it exists. This file is
ignored by git, so you can load it up with anything you want!

Change-Id: I46df8d68dda87fee3e20db1b06f2b771d41d08bb

.gitignore

@@ -127,3 +127,6 @@ toolchain/toolchain.tar.bz2
 # Nix outputs
 /result
 /dev-profile*
+
+# Developer-local justfile
+justfile.local

justfile

@@ -5,6 +5,11 @@
 # - https://github.com/casey/just
 # - https://just.systems/man/en/
 
+# Import justfile.local, if it exists, so that developers can create their own
+# justfile commands that might not be generally useful enough to commit to the
+# project.
+import? "justfile.local"
+
 export BAZEL_CONFIG_FLAG := if env_var_or_default('CI', '') == "" { "" } else { "--config=ci" }
 
 # Quick-and-dirty Cargo.toml workspace finder for cargo commands.