Fix building for iOS

[keith]

Some projects like envoy-mobile use rules_foreign_cc to build targeting
iOS, so this is useful to validate on CI as part of the macOS test
build. This also fixes a bug where this was broken since
c41020e because the wrong SDKROOT was used

[keith]

So I currently expect this to fail, since compatibility with iOS was broken somewhere in this diff d54c78a...b51f25e which we discovered when updating envoy and envoy-mobile's usage of the rules.

My assumption is that it was caused by c41020e which always assumes SDKROOT is macOS specific. But I haven't been able to fix it yet in my tests even after fixing that. The -isysroot arg cmake produces is always pointing to a macOS SDK

[keith]

cc @jsharpe from the commit above hoping you have some ideas here

[keith]

You can see the current failure here https://buildkite.com/bazel/rules-foreign-cc/builds/3452#a07e12a5-3042-484f-98d2-c60cdbe57a95

[keith]

Ok confirmed that was the issue and pushed a fix here. I was thrown off while trying to reduce because of #769

[keith]

I don't see how this could have affected the Linux python3 setup, but I will try to debug more

[jsharpe]

I don't see how this could have affected the Linux python3 setup, but I will try to debug more

Yes, that's an odd failure; I can't see how this change would have affected the RBE python3 build either.

[keith]

My current assumption is that by changing the workspaces here I'm triggering a build that was previously cached, and somehow rotted, maybe through a docker image update

[keith]

Ok I take it back, it seems to be a workspace ordering issue. I think because of some transitive dependency on zlib

[jsharpe]

Thanks for getting to the bottom of this! It's very useful to have more diverse use cases in the tests

[keith]

❤️ thanks!