Fixes bug in bpkg-install global check

The bug occurred when a "global" section was present in a `packages.json` file but had a value other than "true".
Due to the logic of the check, such a package would then be marked for global install.

lib/install/install.sh

@@ -341,7 +341,7 @@ bpkg_install_from_remote () {
     )"
 
     ## check if forced global
-    if [[ ! -z "$(echo -n "${json}" | bpkg-json -b | grep '\["global"\]' | awk '{ print $2 }' | tr -d '"')" ]]; then
+    if [[ "$(echo -n "${json}" | bpkg-json -b | grep '\["global"\]' | awk '{ print $2 }' | tr -d '"')" == 'true' ]]; then
       needs_global=1
     fi