Add unsupported --legacy flag for targeting Unreal Engine 4 #77

Job	Run time
Test StringEnum Trimmed	28s
Timestamp	10s
Build Mac Logging	31s
Build AutoDiscovery	54s
Build macOS UEFS	1m 29s
Build Windows UEFS	3m 2s
Build Libraries on macOS	5m 18s
Build and Test Libraries on Windows	8m 56s
Build macOS Pass 1	4m 50s
Build Windows Pass 1	7m 50s
Build macOS Shim	39s
Build Windows Shim	1m 8s
Build macOS Pass 2	4m 49s
Build Windows Pass 2	7m 50s
Publish UEFS Container	0s
Publish to GitHub	0s
Publish to NuGet	0s
	47m 54s

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