CI (Buildkite): use a TMPDIR that is backed by real storage (JuliaLang#42995)

(cherry picked from commit f317d57)

Author: DilumAluthge

.buildkite/pipelines/main/platforms/tester_linux.yml

@@ -61,6 +61,11 @@ steps:
       unset JULIA_DEPOT_PATH
       unset JULIA_PKG_SERVER
 
+      # Make sure that temp files and temp directories are created in a location that is
+      # backed by real storage.
+      export TMPDIR="$(pwd)/tmp"
+      mkdir -p $${TMPDIR:?}
+
       export NETWORK_RELATED_TESTS="Artifacts Downloads download LazyArtifacts LibGit2/online Pkg"
 
       if [[   "${GROUP?}" == "all" ]]; then