nix: add "cd $rootdir" to all shell scripts via checked-shell-script

nix/overlays/checked-shell-script/checked-shell-script.nix

@@ -1,11 +1,12 @@
 # Create a bash script that is checked with shellcheck. You can either use it
 # directly, or use the .bin attribute to get the script in a bin/ directory,
 # to be used in a path for example.
-{ writeTextFile
-, runtimeShell
+{ git
 , runCommand
-, stdenv
+, runtimeShell
 , shellcheck
+, stdenv
+, writeTextFile
 }:
 name: text:
 let
@@ -20,6 +21,14 @@ let
           #!${runtimeShell}
           set -euo pipefail
 
+          cd "$(${git}/bin/git rev-parse --show-toplevel)"
+
+          if test ! -f postgrest.cabal; then
+            >&2 echo "Couldn't find postgrest.cabal. Please make sure to" \
+                     "run this command somewhere in the PostgREST repo."
+            exit 1
+          fi
+
           ${text}
         '';