remove fetchGit allRefs flag for nix <2.4

Prevent use of allRefs when nix <2.4 is in use.
nix <2.4 does not accept the `allRefs` flag, but has `allRefs` behavior
by default.  Note that this goes beyond the the initial intent of this
PR because it enables naersk's `gitAllRefs` flag to work on nix <2.4.

lib.nix

@@ -111,7 +111,7 @@ rec
           ref = lock.branch;
         } // lib.optionalAttrs (lock ? tag) {
           ref = lock.tag;
-        } // lib.optionalAttrs (gitAllRefs || lock ? rev) {
+        } // lib.optionalAttrs ((lib.versionAtLeast builtins.nixVersion "2.4") && (gitAllRefs || lock ? rev)) {
           allRefs = true;
         } // lib.optionalAttrs gitSubmodules {
           submodules = true;