ci: Use FDO_CI_CONCURRENT as our -j flags when present in the runner env.

fd.o has retuned the x86 runners on packet for -j8.  Rather than having to
tweak our CI every time fd.o decides to rebalance job concurrency, respect
what the runner admin has chosen for their builds (this will also be
convenient for people with large local runners).

Reviewed-by: Michel Dänzer <michel@daenzer.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5669>

.gitlab-ci/container/container_pre_build.sh

@@ -14,16 +14,11 @@ export CXX="/usr/lib/ccache/g++"
 
 ccache --show-stats
 
-if uname -m | grep -q arm || uname -m | grep -q aarch64; then
-    export JFLAGS=-j8
-else
-    export JFLAGS=-j4
-fi
-
 # Make a wrapper script for ninja to always include the -j flags
-echo /usr/bin/ninja $JFLAGS '"$@"' > /usr/local/bin/ninja
+echo '#!/bin/sh -x' > /usr/local/bin/ninja
+echo '/usr/bin/ninja -j${FDO_CI_CONCURRENT:-4} "$@"' >> /usr/local/bin/ninja
 chmod +x /usr/local/bin/ninja
 
 # Set MAKEFLAGS so that all make invocations in container builds include the
 # flags (doesn't apply to non-container builds, but we don't run make there)
-export MAKEFLAGS=$JFLAGS
+export MAKEFLAGS="-j${FDO_CI_CONCURRENT:-4}"