Fix GitHub Actions for macOS

[dra27]

The default macOS runners are now Ventura (14) which has added a realpath utility which causes brew's coreutils package to install realpath as grealpath. Follow brew's configuration advice for the PATH variable.

[kit-ty-kate]

why do we need coreutils in CI on macOS?

[kit-ty-kate]

answering to myself: the reftests

diff --git a/_build/default/tests/reftests/archive.test b/_build/default/tests/reftests/archive.out
index 69c2eaa..58ff033 100644
--- a/_build/default/tests/reftests/archive.test
+++ b/_build/default/tests/reftests/archive.out
@@ -938,7 +938,9 @@ The following actions will be performed:
 -> installed good-sha256-good-md5.1
 Done.
 ### sh check-cache.sh
-MD5: link, to sha256 archive
+realpath: illegal option -- m
+usage: realpath [-q] [path ...]

I feel like we could get rid of that usage instead so we're sure we're testing macOS correctly the way users actually use it

[dra27]

Sure - in the meantime, we ought to get CI back up and running, though!

[dra27]

Although there are two caveats to possibly not changing it: firstly, prior to Ventura IIUC, macOS did not have a realpath utility (it's just added it), but also the usage here is only (I think) in a checking function (i.e. the use of realpath is part of the test harness, not part of the opam being tested)

[kit-ty-kate]

fair enough. I opened a TODO in #5940 to think about that later