gitlab: install flux-core

Problem: We create an install directory, but seem to miss
actually installing flux-core.

Install flux-core so future projects can build with it.
Utilize the installation, rather than the build, in MPI testing.

.gitlab-ci.yml

@@ -70,7 +70,7 @@ default:
     script:
         - export MPI_TESTS_DIRECTORY=$(pwd)/mpi
         - export FTC_DIRECTORY=$(pwd)
-        - flux run -N2 $CORE_BUILD_DIR/src/cmd/flux start $MPI_TESTS_DIRECTORY/outer_script.sh
+        - flux run -N2 $CORE_INSTALL_PREFIX/bin/flux start $MPI_TESTS_DIRECTORY/outer_script.sh
 
 corona-core-build:
     extends: 

.gitlab/builds.gitlab-ci.yml

@@ -27,6 +27,7 @@
         - ../configure --prefix=${CI_DIRECTORY}/flux-core/install
         - make -j $(nproc)
         ## need to install to get pkgconfig files
+        - make -j $(nproc) install
         - export CORE_BUILD_DIR=$(pwd)
         - export CORE_INSTALL_PREFIX=${CI_DIRECTORY}/flux-core/install
         - cd $FTC_DIRECTORY