feat(cheddar): Add a build script that injects BAT_SYNTAXES r/2557

The BAT_SYNTAXES environment variable is injected during Nix builds
and points to the build result of //third_party/bat_syntaxes, which is
included in the cheddar binary.

During development this is a bit annoying: Opening a Nix shell will
set the right variable, but it will also mess with the Cargo
environment in ways that make it annoying to use (*and* require a
pre-build of all dependencies as required by naersk).

For an easier development flow, this adds a build script that
automatically runs Nix, fetches the path to the syntax set and sets
the environment variable. It is a no-op if the variable is already
set.

This makes for a smoother development experience when working on
cheddar in a depot checkout.

Change-Id: Ib5435518897e3443247edd9f362821c0b9ff756b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3081
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

0 files changed, 0 insertions, 0 deletions