Build: Enable incremental build of TF-M SPE

Set BUILD_ALWAYS to ON when building TF-M SPE via ExternalProject_Add().
Enable incremental build of TF-M SPE under spe build directory of
regression tests and PSA arch tests.

Signed-off-by: David Hu <david.hu@arm.com>
Change-Id: Ia9e1f028dfe1d6054f0308483c58e90fc75f8667
2 files changed