File Logger ignores value of APPEND parameter

[rainersigwald]

See https://github.com/MicrosoftDocs/visualstudio-docs/issues/5508. Only the presence of the APPEND parameter to the logger matters when setting append, so append=false means the same thing as append=true.

https://github.com/microsoft/msbuild/blob/6019483f9105a83f8a5bf1fb781ff4a5091f1df5/src/Build/Logging/FileLogger.cs#L195-L197