[CI] Use Spack OCI build cache for MPI packages (#788)

* [CI] Use Spack OCI build cache for MPI packages

* [CI] Use full path of libmpi

* [CI] Delete Spack environment

* Revert "[CI] Use full path of libmpi"

This reverts commit 211c743.

* [CI] Use OCI cache for Intel MPIs

* [CI] Also for Intel MPI + MPITrampoline

* [CI] Create symlink to work around GHA shenanigans

* [CI] Use MPITrampoline with Intel oneAPI MPI

Intel MPI is currently broken, better to use oneAPI MPI.

* [CI] Run MPITrampoline + oneAPI tests on Julia v1

* [CI] Fix `setup-julia` action to v1

* [CI] Remove symlink workaround

Now we have Spack build cache in `/opt/spack`, so that we shouldn't hit the
GitHub Action issue.

* [CI] Ignore failures uploading coverage

Author: giordano