ci: Switch to MPICH in Fedora jobs

The OpenMPI package in Fedora 34 requires a CPU with AVX instructions.
Some of our CI machines do not have them, and so fail the FindMPI.Test
test with `SIGILL`.  Switch to MPICH.  Since we test with OpenMPI on the
Debian jobs, this covers more MPI vendors anyway.

.gitlab/ci/env_fedora34_makefiles.cmake

@@ -1,2 +1,2 @@
 set(ENV{MY_RUBY_HOME} "/usr/local/rvm/rubies/ruby-2.7.0")
-set(ENV{PATH} "/usr/lib64/openmpi/bin:$ENV{PATH}")
+set(ENV{PATH} "/usr/lib64/mpich/bin:$ENV{PATH}")

.gitlab/os-linux.yml

@@ -69,7 +69,7 @@
 ### Fedora
 
 .fedora34:
-    image: "kitware/cmake:ci-fedora34-x86_64-2021-05-10"
+    image: "kitware/cmake:ci-fedora34-x86_64-2021-05-21"
 
     variables:
         GIT_CLONE_PATH: "$CI_BUILDS_DIR/cmake ci/long file name for testing purposes"