Missing OSX support for 2.18

[h-vetinari]

Currently there are no 2.18 builds for osx-arm64 & osx-64. While we believe the recipe would be ready, the builds time out in azure (hard 6h timelimit), and so far there has been no-one that could build this locally according to CFEP-03 - see some discussion in #414.

If you have a MacOS machine and are willing to build the recipe, please let us know!

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In terms of alternatives, I've started a conversation within conda-forge/core to set up a macOS server without a 6h time limit (along the lines of how this now works through cirun for both linux & windows), but this needs funding or sponsorship, as well as some technical integration work, so don't expect this to happen quickly.

As a long shot, it's technically be possible to cross-compile osx-{64,arm64} from a linux-64 host, but from attempting this in #415, it is an absolute nightmare with bazel (despite providing what I understand to be the correct configuration, it refuses to respect the necessarily non-default location of the downloaded macOS SDK).