builddir not set correctly when build_in_installdir = True and running --sanity-check-only

[branfosj]

When self.build_in_installdir = True (c.f. OpenFOAM easyblock) and running --sanity-check-only then self.builddir is not set correctly.

Error:

cd: /dev/shm/build-branfosj-admin/branfosj-admin-up/OpenFOAM/8/foss-2020b: No such file or directory

That corresponds to

buildpath                 (E) = /dev/shm/build-branfosj-admin/branfosj-admin-up

[boegel]

The problem is that self.builddir = self.installdir is only set in make_builddir, which is part of the ready step (that is skipped entirely when using --sanity-check-only).

Fixed in #3900 by moving self.builddir = self.installdir to the EasyBlock constructor, so it's always done, regardless of which steps are run.