Make flake evaluation resilient to missing self.rev

The attribute self.rev is missing also when flake inputs are overridden,
and it's highly desirable that the flake's evaluation is able to succeed
with overridden inputs.

The workbench-run-supervisor-ci-test-bage has its own logic around
checking self.rev, which does not use the top-level overlay; and the
system-tests script aborts the evaluation when self.rev is missing.

This change makes workbench-run-supervisor-ci-test-bage use pkgs.gitrev
as the rest of the flake and moves the checking self.rev to inside
system-tests script.

Author: andreabedini

flake.nix

@@ -206,10 +206,7 @@
                     inherit profileName workbenchStartArgs;
                     backendName = "supervisor";
                     useCabalRun = false;
-                    cardano-node-rev =
-                      if builtins.hasAttr "rev" self
-                      then pkgs.gitrev
-                      else throw "Cannot get git revision of 'cardano-node', unclean checkout?";
+                    cardano-node-rev = pkgs.gitrev;
                   }).workbench-profile-run;
           in
           {
@@ -231,7 +228,11 @@
               name = "system-tests";
               runtimeInputs = with pkgs; [ git gnused ];
               text = ''
-                  NODE_REV="${self.rev or (throw "Sorry, need clean/pushed git revision to run system tests")}"
+                  NODE_REV="${self.rev or ""}"
+                  if [[ -z $NODE_REV ]]; then
+                    echo "Sorry, need clean/pushed git revision to run system tests"
+                    exit 1;
+                  fi
                   MAKE_TARGET=testpr
                   mkdir -p tmp && cd tmp
                   rm -rf cardano-node-tests