ci: Not set 'KATA_DEV_MODE' to 'false' explicitly

In many of our CI scripts, we assume the 'KATA_DEV_MODE' environment
variable to be not set at all when we are not in the 'kata dev
mode'. Setting its value explicitly to 'false' would fail the check '[-n
$KATA_DEV_MODE]' and make the script believe we ARE in the 'kata dev
mode', which would skip some installation. For example,
'install_rust.sh' is totally skipped in CLH's CI, which caused our
current regression.

Fixes: #2863

Signed-off-by: Bo Chen <chen.bo@intel.com>

.ci/jenkins_job_build.sh

@@ -27,7 +27,12 @@ init_ci_flags() {
 	# Make jobs work like in CI
 	# CI disables non-working tests
 	export CI="true"
-	export KATA_DEV_MODE="false"
+
+	# Many checks assume this environment variable to be not set
+	# (e.g. [ -n $KATA_DEV_MODE ]). As a result, even its value is
+	# set to 'false', the check would think we are in "kata_dev_mode".
+	# export KATA_DEV_MODE="false"
+
 	# Install crio
 	export CRIO="no"
 	# Install cri-containerd