Allow specifying alternative paths for reading/writing flake locks

This allows having multiple separate lockfiles for a single
project, which can be useful for testing against different versions of
nixpkgs; it also allows tracking custom input overrides for remote
flakes without requiring local clones of these flakes.

For example, if I want to build Nix against my locally pinned nixpkgs,
and have a lock file tracking this override independently of future
updates to said nixpkgs:

nix flake lock --output-lock-file /tmp/nix-flake.lock --override-input nixpkgs flake:nixpkgs
nix build --reference-lock-file /tmp/nix-flake.lock

Co-Authored-By: Will Fancher <elvishjerricco@gmail.com>

src/libcmd/installables.cc

@@ -102,6 +102,28 @@ MixFlakeOptions::MixFlakeOptions()
         }}
     });
 
+    addFlag({
+        .longName = "reference-lock-file",
+        .description = "Read the given lock file instead of `flake.lock` within the top-level flake",
+        .category = category,
+        .labels = {"flake-lock-path"},
+        .handler = {[&](std::string lockFilePath) {
+            lockFlags.referenceLockFilePath = lockFilePath;
+        }},
+        .completer = completePath
+    });
+
+    addFlag({
+        .longName = "output-lock-file",
+        .description = "Write the given lock file instead of `flake.lock` within the top-level flake",
+        .category = category,
+        .labels = {"flake-lock-path"},
+        .handler = {[&](std::string lockFilePath) {
+            lockFlags.outputLockFilePath = lockFilePath;
+        }},
+        .completer = completePath
+    });
+
     addFlag({
         .longName = "inputs-from",
         .description = "Use the inputs of the specified flake as registry entries.",