nix: override PGRST_CMD for postgrest-loadtest from env

Enables `PGRST_CMD=postgrest-profiled-run postgrest-loadtest` running
loadtest against profiled executable, which wasn't available before

nix/tools/withTools.nix

@@ -361,6 +361,7 @@ let
             "ARG_POSITIONAL_SINGLE([command], [Command to run])"
             "ARG_LEFTOVERS([command arguments])"
             "ARG_OPTIONAL_SINGLE([monitor], [m], [Enable CPU and memory monitoring of the PostgREST process and output to the designated file as markdown])"
+            "ARG_USE_ENV([PGRST_CMD], [], [PostgREST executable to run])"
           ];
         positionalCompletion = "_command";
         workingDir = "/";
@@ -370,6 +371,7 @@ let
       ''
         export PGRST_SERVER_UNIX_SOCKET="$tmpdir"/postgrest.socket
 
+        if [ -z "''${PGRST_CMD:-}" ]; then
           rm -f result
           if [ -z "''${PGRST_BUILD_CABAL:-}" ]; then
             echo -n "Building postgrest (nix)... "
@@ -386,6 +388,7 @@ let
             PGRST_CMD=postgrest-run
           fi
           echo "done."
+        fi
 
         ver=$($PGRST_CMD ${legacyConfig} --version)