Pull linode openapi spec from a fixed version in github, and so the
sha256 hash will not break on each new release of the api spec.

Replaced the hash for the openapi spec with a working hash.

Attempting to install linode-cli was failing, with the following
message:

hash mismatch in fixed-output derivation '/nix/store/px3cdbcb84bvw6xslr1k6hszyxpdis6j-openapi.yaml':
  wanted: sha256:1l2fahdcmv7sp1qkwr5nv2vls8fypvlybwylqfzhyjmn7jqkw4hq
  got:    sha256:03ngzbq24zazfqmfd7xjmxixkcb9vv1jgamplsj633j7sjj708s0
cannot build derivation '/nix/store/49xrq47id66kwszyaqg1qapknc3i8mmx-linode-cli-2.14.1.drv': 1 dependencies couldn't be built