Fix iOS simulator CI (#323)

The upgrade to macOS 12 broke this iOS simulator CI (as support for the
iPhone 7 was dropped I think).

We should have a follow up PR to see if we can just test against a more
recent iPhone.

Signed-off-by: Joe Richey <[email protected]>

Author: josephlr

.github/workflows/tests.yml

@@ -85,7 +85,7 @@ jobs:
 
   ios-tests:
     name: iOS Simulator Test
-    runs-on: macos-latest
+    runs-on: macos-11
     steps:
       - uses: actions/checkout@v3
       - uses: dtolnay/rust-toolchain@stable