ci/parse: only show first error

There is no point in running the much slower `parse-each` part for each
interpreter/version. The CI job is not meant as a development tool that
should report all parse errors at once, but as a confirmation that no
parse errors are present on *different interpreter versions*.

Once this test fails, Eval, nixpkgs-vet and treefmt will most likely
fail as well - with more information for multiple parse errors.

ci/parse.nix

@@ -23,21 +23,11 @@ runCommand "nix-parse-${nix.name}"
 
     cd "${nixpkgs}"
 
-    # Passes all files to nix-instantiate at once.
-    # Much faster, but will only show first error.
-    parse-all() {
-      find . -type f -iname '*.nix' | xargs -P $(nproc) nix-instantiate --parse >/dev/null 2>/dev/null
-    }
-
-    # Passes each file separately to nix-instantiate with -n1.
-    # Much slower, but will show all errors.
-    parse-each() {
-      find . -type f -iname '*.nix' | xargs -n1 -P $(nproc) nix-instantiate --parse >/dev/null
-    }
-
-    if ! parse-all; then
-      parse-each
-    fi
+    # This will only show the first parse error, not all of them. That's fine, because
+    # the other CI jobs will report in more detail. This job is about checking parsing
+    # across different implementations / versions, not about providing the best DX.
+    # Returning all parse errors requires significantly more resources.
+    find . -type f -iname '*.nix' | xargs -P $(nproc) nix-instantiate --parse >/dev/null
 
     touch $out
   ''