Make it easy to invoke Hadrian

Right now, invoking Hadrian requires running hadrian/build.sh. That's a small mouthful (when using it regularly), and it crucially depends on being in the root directory of GHC. It would be very convenient to have some command (say, build) which would work anywhere within the GHC tree, e.g. by searching for the root and then running the local Hadrian build script. Doing this would require devs to install something locally (so that it's in the PATH always), but that something can be small and light.

Shall we fix? Thanks.

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Build System (Hadrian)
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