Fix postgres12 git reference commit

[teutat3s]

While testing manatee with postres12 I noticed that the tests setup was somehow broken and tracked it down to upstream changes. It seems that the commit used for referencing and building postgres12 in https://github.com/joyent/manatee/blob/84fac098c7656bb3b735f2fe0663fd09e2706942/tools/mkdevsitters#L16 is no longer available in any upstream branch.

To fix this, this PR suggests using the commit for the release tag 12.3.

Fixes #30.