Use /opt/local/bin/git

The osx:ansi-test fails with this message:
```
$ git clone https://gitlab.common-lisp.net/cmucl/ansi-test.git
Cloning into 'ansi-test'...
fatal: unable to access
'https://gitlab.common-lisp.net/cmucl/ansi-test.git/': SSL certificate problem: certificate has expired
```

But I don't know why.  I can run this manually using /usr/bin/git and
/opt/local/bin/git without running into the problem.  Trying
/opt/local/bin/git to see if it helps.
10 jobs for issue-119-ci-osx-install-fails in 31 minutes and 50 seconds (queued for 6 seconds)
latest
Status Name Job ID Coverage
  Install
passed linux:install #21830
linux

00:02:18

passed osx:install #21831
osx

00:01:29

 
  Build
passed linux:build #21832
linux

00:07:02

passed osx:build #21833
osx

00:11:13

 
  Test
passed linux:test #21834
linux

00:00:57

passed osx:test #21835
osx

00:02:54

 
  Ansi Test
passed linux:ansi-test #21836
linux

00:03:47

passed osx:ansi-test #21837
osx

00:06:29

 
  Benchmark
passed linux:benchmark #21838
linux

00:07:20

passed osx:benchmark #21839
osx

00:09:41