ci: Pin MacPorts version to 2.9.3.

ci: Pin MacPorts version to 2.9.3.

Commit d01ce180 invented a new way to find the latest MacPorts version.
By bad luck, a new beta release has just been published, and it seems
to lack some packages we need.  Go back to searching for this specific
version for now.  We still search with a pattern so that we can find the
package for the running version of macOS, but for now we always look for
2.9.3.  The code to do that had been anticipated already in a commented
out line, I just didn't expect to have to use it so soon...

Also include the whole MacPorts installation script in the cache key, so
that changes to the script cause a fresh installation.  This should make
it a bit easier to reason about the effect of changes on cached state in
github accounts using CI, when we make adjustments.

Back-patch to 15, like d01ce180.

Discussion: https://postgr.es/m/CA%2BhUKGLqJdv6RcwyZ_0H7khxtLTNJyuK%2BvDFzv3uwYbn8hKH6A%40mail.gmail.com