project planning: fix #10686 regression

We previously only fetched the compiler in
`instantiateProjectConfigSkeletonFetchingCompiler` if
`null (toListOf traverseCondTreeV skel)` was false.

c89ab54e changed this to always
configure the compiler, such that it could be later re-used when
determining whether jsem was supported to clone git submodules
concurrently.

This patch allows for the JobControl concurrency mechanism to be
controlled without a compiler (which assumes -jsem is not supported),
and restores the behaviour of not always configuring the compiler.

Fixes #10686

(cherry picked from commit b817cb7a)

# Conflicts:
#	cabal-install/src/Distribution/Client/CmdInstall.hs
#	cabal-install/src/Distribution/Client/JobControl.hs
#	cabal-install/src/Distribution/Client/ProjectBuilding.hs
#	cabal-install/src/Distribution/Client/ProjectConfig.hs
#	cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs
#	cabal-install/src/Distribution/Client/ProjectPlanning.hs
#	cabal-install/src/Distribution/Client/ScriptUtils.hs