project planning: fix #10686 regression
We previously only fetched the compiler in
`instantiateProjectConfigSkeletonFetchingCompiler` if
`null (toListOf traverseCondTreeV skel)` was false.
c89ab54e13f1eb808dcdd470edecd7eaaa594cec 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