Hadrian should use _build/ subfolder based on --flavour

The easy to reproduce problem:

./hadrian/build.cabal.sh  # succeeds
./hadrian/build.cabal.sh  --flavour=quick # fails

Error when running Shake build system:
  at action, called at src/Rules.hs:69:19 in main:Rules
  at need, called at src/Rules.hs:91:5 in main:Rules
* Depends on: _build/stage1/lib/package.conf.d/rts-1.0.conf
  at need, called at src/Rules/Rts.hs:31:9 in main:Rules.Rts
* Depends on: _build/stage1/lib/x86_64-linux-ghc-8.9.0.20190507/libHSrts-ghc8.9.0.20190507.so
* Raised the exception:
_build/stage1/lib/x86_64-linux-ghc-8.9.0.20190507/libHSrts-ghc8.9.0.20190507.so: createFileLink:createSymbolicLink: already exists (File exists)

I think there isn't much non-rebuildable stuff, so having _build subdir based on flavour (and maybe other similar, largely altering options) seems fair.

04:04  angerman o/                                                                                                                                                                                   
04:06  angerman phadej: for different flavours/configurations, you want to use different build roots. The default one is _build, but you can set that via a command line flag when invoking hadrian.