rpmbuild: properly set root dir and enable tracing (#1272)

As git_checkout() is not popping from the checkout dir we were setting
the root directory inside the package's source code (not the expected),
for that then we set the variable before the first git_checkout() call.

Additionally we are setting -e and -x flags so the it fails early and
also prints the script tracing.

packagebuild/daisy_startupscript_rpm.sh

@@ -18,6 +18,8 @@
 # repository to clone. The script produces an RPM defined by an RPM spec in the
 # packaging/ directory from the cloned repo.
 
+set -ex
+
 URL="http://metadata/computeMetadata/v1/instance/attributes"
 function get_md() {
   curl -f -H Metadata-Flavor:Google "${URL}/${1}"
@@ -76,8 +78,8 @@ fi
 
 try_command yum install -y $GIT rpmdevtools yum-utils python3-devel
 
-git_checkout "$REPO_OWNER" "$REPO_NAME" "$GIT_REF"
 ROOT_WORK_DIR=$(pwd)
+git_checkout "$REPO_OWNER" "$REPO_NAME" "$GIT_REF"
 
 if [[ -n "$BUILD_DIR" ]]; then
   cd "$BUILD_DIR"