7471: Force XCode version r=lnicola a=lnicola
Pin an Xcode version in the release workflow to avoid:
```
Run SDKROOT=$(xcrun -sdk macosx11.0 --show-sdk-path) MACOSX_DEPLOYMENT_TARGET=$(xcrun -sdk macosx11.0 --show-sdk-platform-version) cargo xtask dist
xcodebuild: error: SDK "macosx11.0" cannot be located.
xcodebuild: error: SDK "macosx11.0" cannot be located.
xcrun: error: unable to lookup item 'Path' in SDK 'macosx11.0'
xcodebuild: error: SDK "macosx11.0" cannot be located.
xcodebuild: error: SDK "macosx11.0" cannot be located.
xcrun: error: unable to lookup item 'PlatformVersion' in SDK 'macosx11.0'
```
Co-authored-by: Laurențiu Nicola <lnicola@dend.ro>