Update iOS Simulator OS version to 12.4

Summary:
It looks like Sandcastle is now using version iOS v12.4 for the iOS simulator. This is causing `test-react-native-oss-ios-legocastle.sh` to fail. See [this build failure](https://our.intern.facebook.com/intern/sandcastle/job/31525197549441452/).

```
Failure Found: react-native-oss-ios - Test: Run RNTesterPods Integration Tests
stdout:
User defaults from command line:
    IDETestRunOnlyIdentifiers = (
    RNTesterUnitTests
)
    IDETestRunSkipIdentifiers = (
    "RNTesterIntegrationTests/RNTesterSnapshotTests",
    "RNTesterIntegrationTests/RNTesterIntegrationTests/testWebSocketTest"
)
Build settings from command line:
    RN_BUNDLE_PREFIX = react-native-github/
    SDKROOT = iphonesimulator12.4
stderr:
xcodebuild: error: Unable to find a destination matching the provided destination specifier:
		{ platform:iOS Simulator, OS:12.2, name:iPhone 6s }
	The requested device could not be found because no available devices matched the request.
	Available destinations for the "RNTester" scheme:
		{ platform:iOS Simulator, id:5E437CA6-5FB6-4AB3-BF7B-F04B2BEACE3A, OS:12.4, name:iPad (5th generation) }
		{ platform:iOS Simulator, id:054F7E3D-F1BC-48BF-8C0E-FD047CD8145D, OS:12.4, name:iPad (6th generation) }
		{ platform:iOS Simulator, id:FF1DE07E-6DB8-43DB-AF89-A788054639EB, OS:12.4, name:iPad Air }
		{ platform:iOS Simulator, id:257E7F9D-436C-4D3F-934F-AF87C3FD9C57, OS:12.4, name:iPad Air (3rd generation) }
		{ platform:iOS Simulator, id:8CA6B68A-A191-406A-B75C-6EA61762E503, OS:12.4, name:iPad Air 2 }
		{ platform:iOS Simulator, id:2250ACFE-B7C9-47F8-AB9D-76BCAD2EA623, OS:12.4, name:iPad Pro (9.7-inch) }
		{ platform:iOS Simulator, id:89BA7B06-DA47-4EB3-8154-282BC9606884, OS:12.4, name:iPad Pro (10.5-inch) }
		{ platform:iOS Simulator, id:5AFD752C-7994-4343-8936-3045D4EA4098, OS:12.4, name:iPad Pro (11-inch) }
		{ platform:iOS Simulator, id:BE6D8220-5E32-47DB-9403-91F4649851EA, OS:12.4, name:iPad Pro (12.9-inch) }
		{ platform:iOS Simulator, id:820B2496-B679-4EE9-950B-826BF129817E, OS:12.4, name:iPad Pro (12.9-inch) (2nd generation) }
		{ platform:iOS Simulator, id:0F257958-76B4-43B8-BA9D-C7E2C1137D19, OS:12.4, name:iPad Pro (12.9-inch) (3rd generation) }
		{ platform:iOS Simulator, id:CAF47FC7-A1A8-4503-958E-F6A3094650C4, OS:12.4, name:iPad mini (5th generation) }
		{ platform:iOS Simulator, id:855BEDAC-6E5F-4638-85B3-B52A538CCB44, OS:12.4, name:iPad mini 2 }
		{ platform:iOS Simulator, id:C0060E26-DBC0-4904-B60A-825FA610BA12, OS:12.4, name:iPad mini 3 }
		{ platform:iOS Simulator, id:5A56D7DD-14A7-4E1F-B672-969C67D0BC5D, OS:12.4, name:iPad mini 4 }
		{ platform:iOS Simulator, id:17E11CDB-62CD-4CFA-827C-9499233845AF, OS:12.4, name:iPhone 5s }
		{ platform:iOS Simulator, id:9D0E91A4-B302-4262-A4B8-700D77187739, OS:12.4, name:iPhone 6 }
		{ platform:iOS Simulator, id:3A02CF93-4216-4294-BCA4-8D373DEA773F, OS:12.4, name:iPhone 6 Plus }
		{ platform:iOS Simulator, id:8B07790E-16E6-4469-9523-4AD7A7C89683, OS:12.4, name:iPhone 6s }
		{ platform:iOS Simulator, id:097F6248-9244-487C-B0E3-D4BA1878B2A6, OS:12.4, name:iPhone 6s Plus }
		{ platform:iOS Simulator, id:450708A9-08E0-48A4-8581-B0B13AE5B705, OS:12.4, name:iPhone 7 }
		{ platform:iOS Simulator, id:C6A991EC-6A3A-4E9F-91F5-701F72594E00, OS:12.4, name:iPhone 7 Plus }
		{ platform:iOS Simulator, id:5F40F20C-88F4-4C00-A793-916CE9F6AB36, OS:12.4, name:iPhone 8 }
		{ platform:iOS Simulator, id:464700B3-E305-4A89-9FEC-462FC157FBF2, OS:12.4, name:iPhone 8 Plus }
		{ platform:iOS Simulator, id:66C1DB35-A629-4F2F-A312-FF589E0CEF7F, OS:12.4, name:iPhone SE }
		{ platform:iOS Simulator, id:71121451-3E76-465F-9DDC-26B935A7828B, OS:12.4, name:iPhone X }
		{ platform:iOS Simulator, id:1E0BE5F8-2075-4E19-A9F1-A307E0EF4808, OS:12.4, name:iPhone Xs }
		{ platform:iOS Simulator, id:646D550E-770B-4D00-84B1-BF711C978E80, OS:12.4, name:iPhone Xs Max }
		{ platform:iOS Simulator, id:B0511543-649C-43E8-9782-17C559C19E65, OS:12.4, name:iPhone Xʀ }
	Ineligible destinations for the "RNTester" scheme:
		{ platform:iOS, id:dvtdevice-DVTiPhonePlaceholder-iphoneos:placeholder, name:Generic iOS Device }
		{ platform:iOS Simulator, id:dvtdevice-DVTiOSDeviceSimulatorPlaceholder-iphonesimulator:placeholder, name:Generic iOS Simulator Device }
```

This diff updates the version to 12.4, as suggested by the above logs.

build-break
overriding_review_checks_triggers_an_audit_and_retroactive_review

Differential Revision: D18218399

fbshipit-source-id: 6625f94f249721396ef82641f628f399ebfd5302

scripts/.tests.env

@@ -17,7 +17,7 @@ export AVD_NAME="testAVD"
 export AVD_ABI=x86
 
 ## IOS ##
-export IOS_TARGET_OS="12.2"
+export IOS_TARGET_OS="12.4"
 export IOS_DEVICE="iPhone 6s"
 export TVOS_DEVICE="Apple TV"