ci: disable command execution in jobset generation

When GitLab PR descriptions contain markdown inline code blocks they get
interpreted as command substitutions in bash.

This is because the here-doc string previously allowed for this behavior.

.hydra/declarative-jobsets.nix

@@ -43,11 +43,11 @@ let
 in
 {
   jobsets = pkgs.runCommand "spec-jobsets.json" { } ''
-    cat >$out <<EOF
+    cat >$out <<'EOF'
     ${builtins.toJSON desc}
     EOF
     # This is to get nice .jobsets build logs on Hydra
-    cat >tmp <<EOF
+    cat >tmp <<'EOF'
     ${builtins.toJSON log}
     EOF
     ${pkgs.jq}/bin/jq . tmp