Always invoke gradle except as root

The custom target used to invoke Gradle from Meson should always
be built, otherwise, the server would not be rebuilt on source changes.

However, when enabling "build_always", gradle is invoked as root on
"sudo ninja install" after "ninja", so it downloads the whole Gradle
world into /root/.gradle.

To avoid the problem, just do not call gradle if the effective user id
is 0.

server/scripts/build-wrapper.sh

@@ -2,6 +2,16 @@
 # Wrapper script to invoke gradle from meson
 set -e
 
+# Do not execute gradle when ninja is called as root (it would download the
+# whole gradle world in /root/.gradle).
+# This is typically useful for calling "sudo ninja install" after a "ninja
+# install"
+if [[ "$EUID" == 0 ]]
+then
+    echo "(not invoking gradle, since we are root)" >&2
+    exit 0
+fi
+
 PROJECT_ROOT="$1"
 OUTPUT="$2"
 BUILDTYPE="$3"