build(backend): fix broken and slow pip-compile.sh (#5239)

### 💭 Notes
Script was broken due to incomplete removals in #5125 and #5127.
Slow steps of installing `gcc` and `libpq-dev` are unnecessary, even
when upgrading `psycopg`. Internal discussion:
https://chat.kobotoolbox.org/#narrow/stream/4-Kobo-Dev/topic/pip-tools/near/482296

Author: jnm

pip-compile.sh

@@ -1,14 +1,10 @@
 #!/bin/bash
 
-if [ -z "$(dpkg -l|grep 'libpq-dev')" ]; then
-  echo "Installing dependencies needed to pip-compile..."
-  apt-get -qq update && apt-get -qq -y install gcc libpq-dev
-fi
+set -x  # give the human some hope of progress
 
 for in_file in dependencies/pip/*.in
 do
     # pass any arguments to pip-compile
     # useful for switches like `--upgrade-package`
     pip-compile "$@" "$in_file" || exit $?
 done
-for out_file in dependencies/pip/*.txt