tests: Delete build directory before running KVM in nightly

The nightly tests failed here:
https://jenkins.gem5.org/job/nightly/430/. What seems to have happened
is the ALL/gem5.opt us compiled within the Docker container but then,
for the KVM tests, there is an attempt to recompile on the host, which
causes compilation problems. The safest strategy here is delete the
build directory prior to running the KVM tests.

In latest versions of our test infrastructure, the KVM tests should be
run completely separately (i.e., in different Jenkin's jobs) to avoid
this.

Change-Id: Id7d18c0504dd324f7a0e5e9a7809463520969dda
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/65911
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>

tests/nightly.sh

@@ -89,6 +89,10 @@ docker run -u $UID:$GID --volume "${gem5_root}":"${gem5_root}" -w \
 # removes all those part of the 'very-long' (weekly) tests, or for compilation
 # to '.debug' or '.fast'. We also remove ARM targets as our Jenkins is an X86
 # system. Users wishing to run this script elsewhere should be aware of this.
+# Note: we delete the build directory here. It was build in the
+# "ubuntu-22.04_all-dependencies" docker image which may not be compatible with
+# the host environment.
+rm -rf "${gem5_root}/build"
 cd "${gem5_root}/tests"
 ./main.py run -j${compile_threads} -vv \
     --exclude-tags ".*" --include-tags kvm --exclude-tags very\-long \