[fx][delta] Invoke pm directly

Invoke pm directly instead of proxying through fx, similar to how
serve-updates invokes it.  When building product configs that don't
include pm, "fx pm" fails to find the binary.

Test: manual
Change-Id: Id585f52de81ca9c1cdd94ec127e5a0f5126c7763
diff --git a/devshell/delta b/devshell/delta
index d7d5ab3..924c851 100755
--- a/devshell/delta
+++ b/devshell/delta
@@ -17,11 +17,13 @@
 source "$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null 2>&1 && pwd)"/lib/vars.sh || exit $?
 fx-config-read
 
+pm_bin="${FUCHSIA_BUILD_DIR}/host_x64/pm"
+
 function usage {
   fx-command-help delta
 
   echo
-  fx pm delta --help
+  "${pm_bin}" delta --help
 }
 
 function main {
@@ -77,7 +79,7 @@
     }
   fi
 
-  fx pm delta "${delta_args[@]}" "${source_path}" "${target_path}"
+  "${pm_bin}" delta "${delta_args[@]}" "${source_path}" "${target_path}"
 }
 
 main "$@"