Use double quotes instead of single.

To get the out_dir expansion.

Change-Id: I04e973db39610858fd2314a13a87a51a80bbbc87
diff --git a/tools/buildbot-build.sh b/tools/buildbot-build.sh
index ba87284..bac0e13 100755
--- a/tools/buildbot-build.sh
+++ b/tools/buildbot-build.sh
@@ -151,7 +151,7 @@
     rm -rf $conscrypt_dir
     mkdir $conscrypt_dir
     if [[ -z "${ANDROID_HOST_OUT}" ]]; then
-      echo 'ANDROID_HOST_OUT environment variable is empty; using $out_dir/host/linux-x86'
+      echo "ANDROID_HOST_OUT environment variable is empty; using $out_dir/host/linux-x86"
       ANDROID_HOST_OUT=$out_dir/host/linux-x86
     fi
     $ANDROID_HOST_OUT/bin/deapexer extract $conscrypt_apex $conscrypt_dir