chore: update vendored opam

I've also modified the vendor script slightly so that we do a more
efficient "git clone" by simply fetch the commit we want straight from
the remote and checking it out locally. Downloading opam takes a long
time, so I think this is a good idea.

We can eventually consolidate a lot of the common funcitons in the
vendor update scripts and do the same everywhere.

Signed-off-by: Ali Caglayan <alizter@gmail.com>

vendor/update-opam.sh

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
-version=dcb5165badc5fe87f7f34518c13b7d8e69a79f6f
+version=0984de6a2def6ddf2b3beab7857c062db7e0319d
 
 set -e -o pipefail