[ci] Add a `.buildkite` entry to `CODEOWNERS`.
[ci] Add a `.buildkite` entry to `CODEOWNERS`.

While not strictly necessary, I suspect those who are more keen on
our release mechanics might want this called out specifically.

Additionally, I added myself to the `tools/` directory.

Signed-off-by: Fletcher Nichol <77a0fd9e8048bbacd11af4e957bc6ff03b549f49@nichol.ca>

rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat