Use fixed iOS simulator runtime version in mac.yml iphone_simulator job (#25794)

### Description
<!-- Describe your changes. -->

Set iOS simulator runtime version to 18.5 in mac.yml iphone_simulator
job.

This job uses Xcode 16.4. According to this table, the corresponding
simulator SDK version is 18.5.

https://github.com/actions/runner-images/blob/da7977bf2699f44e70b7d3c3352dedb0da38db9c/images/macos/macos-15-arm64-Readme.md?plain=1#L181

### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->

Address intermittent CI build timeouts.

Author: edgchen1

.github/workflows/mac.yml

@@ -39,6 +39,7 @@ jobs:
           {"machine": "arm64", "target": "arm64", "build_config": "Debug"},
           {"machine": "arm64", "target": "arm64", "build_config": "Release"}
         ]
+
   xnnpack:
     uses: ./.github/workflows/macos-ci-build-and-test-workflow.yml
     with:
@@ -59,11 +60,13 @@ jobs:
           {"machine": "arm64", "target": "arm64", "build_config": "Debug"},
           {"machine": "arm64", "target": "arm64", "build_config": "Release"}
         ]
+
   iphone_simulator:
     runs-on: macos-15
 
     env:
       xcode_version: 16.4
+      simulator_runtime_version: 18.5
 
     strategy:
       matrix:
@@ -100,6 +103,8 @@ jobs:
             --apple_deploy_target=15.1 \
             --apple_sysroot=iphonesimulator \
             --osx_arch=${{ matrix.target_arch }}
+        env:
+          ORT_GET_SIMULATOR_DEVICE_INFO_REQUESTED_RUNTIME_VERSION: ${{ env.simulator_runtime_version }}
 
   Objective-C-StaticAnalysis:
     runs-on: macos-14