[gn] Don't run a python script unconditionally

Part of https://github.com/flutter/flutter/issues/144430

Change-Id: Ie0d998ed98fc5fe3ad8508809f1495cdd3663c59
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/355548
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Zach Anderson <zra@google.com>

sdk_args.gni

@@ -19,12 +19,7 @@ declare_args() {
   # to out/ReleaseX64/args.gn. The path above can be extracted from the `.git`
   # file under the git worktree folder.
   # The script run here should take care of everything automatically though.
-  default_git_folder = exec_script("$_dart_root/tools/get_dot_git_folder.py",
-                                   [
-                                     rebase_path("$_dart_root/.git"),
-                                     "$_dart_root/.git",
-                                   ],
-                                   "trim string")
+  default_git_folder = ""
 
   # Whether to enable the SDK hash check that will prevent loading a kernel
   # into a VM which was built with a different SDK.
@@ -34,6 +29,15 @@ declare_args() {
   dart_sdk_output = "dart-sdk"
 }
 
+if (default_git_folder == "") {
+  default_git_folder = exec_script("$_dart_root/tools/get_dot_git_folder.py",
+                                   [
+                                     rebase_path("$_dart_root/.git"),
+                                     "$_dart_root/.git",
+                                   ],
+                                   "trim string")
+}
+
 # The SDK hash to build into VM and kernels.
 # The value 0000000000 signifies no hash is set, which will disable the check.
 if (verify_sdk_hash) {