On a fresh checkout, db039a4a

# succeeds
./hadrian/build.sh --progress-colour=Never --share=_share -j3
...

# fails
./hadrian/build.sh --progress-colour=Never --share=_share -j3  --build-root=_build-experiment
...

# this succeeds ok
./hadrian/build.sh --progress-colour=Never --share=_share -j3
...
Build completed in 6.97s

# Trying without share: still fails with the same error
./hadrian/build.sh --progress-colour=Never -j3  --build-root=_build-experiment
...
| Run Ghc CompileHs Stage1: libraries/ghc-prim/GHC/Classes.hs => _build-baseline/stage1/libraries/ghc-prim/build/GHC/Classes.p_o
Error when running Shake build system:
  at action, called at src/Rules.hs:35:19 in main:Rules
  at need, called at src/Rules.hs:52:5 in main:Rules
* Depends on: _build-baseline/stage1/bin/ghc-pkg
  at need, called at src/Utilities.hs:71:18 in main:Utilities
* Depends on: _build-baseline/stage1/libraries/integer-gmp/build/libHSinteger-gmp-1.0.2.0-ghc8.9.20190306.so
  at need, called at src/Rules/Gmp.hs:14:5 in main:Rules.Gmp
* Depends on: _build-baseline/stage1/gmp/include/ghc-gmp.h
  at need, called at src/Development/Shake/Internal/Derived.hs:118:15 in shake-0.17.6-ef5d6f7a92b6a347b5077f2591e0d47990d974d264275e5e4048829d4b393706:Development.Shake.Internal.Derived
* Depends on: _build-baseline/stage1/libraries/integer-gmp/build/config.mk
  at error, called at src/Development/Shake/Internal/Rules/File.hs:180:58 in shake-0.17.6-ef5d6f7a92b6a347b5077f2591e0d47990d974d264275e5e4048829d4b393706:Development.Shake.Internal.Rules.File
* Raised the exception:
Error, file does not exist and no rule available:
  _build-baseline/stage1/libraries/integer-gmp/build/config.mk