CI always rebuilds sandbox root

[leoluk]

When Jenkins cleans up the repo, it also deletes .bazeldnf and .bazelrc.sandbox, forcing the sandbox root to be rebuilt.

This is relatively fast - the RPMs are already cached and the --action_env stays the same, so nothing is invalidated - but it increases no-op CI time to 2m: https://jenkins.monogon.dev/blue/organizations/jenkins/gerrit-presubmit-monogon/detail/07%2F1107%2F1/1/pipeline

+ /home/ci/bazelisk test //...
cat: /home/ci/monogon/tools/../.bazelrc.sandbox: No such file or directory
Regenerating sysroot /home/ci/monogon/tools/../.bazeldnf/sandbox/default ...

Weirdly, it is then rebuilt again for the -c dbg run:

+ /home/ci/bazelisk test -c dbg //...
Regenerating sysroot /home/ci/monogon/tools/../.bazeldnf/sandbox/default ...

It's not clear whether re-using the sandboxroot is guaranteed to be safe (not sure if it's mounted r/o?). If it is, there's no point in clearing it. Perhaps it should be moved out of the repo to ~/.cache?