Fix the remote setup to only change the URL. (#15528)

This changes the packaging tool to only set the remote URL instead of recreating the remote.

Recreating it was removing some metadata that identified the repo as having been cloned from the "real" repo, so flutter --version would report an unknown channel and source. This just sets the URL so that it looks like it came from GitHub.

I also fixed some incorrect comments, and made unzip work on platforms other than Windows (even though we don't really need it there yet).

Fixes #15518
diff --git a/dev/bots/test/prepare_package_test.dart b/dev/bots/test/prepare_package_test.dart
index 91e9405..a6424cc 100644
--- a/dev/bots/test/prepare_package_test.dart
+++ b/dev/bots/test/prepare_package_test.dart
@@ -113,8 +113,7 @@
           'git clone -b dev https://chromium.googlesource.com/external/github.com/flutter/flutter':
               null,
           'git reset --hard $testRef': null,
-          'git remote remove origin': null,
-          'git remote add origin https://github.com/flutter/flutter.git': null,
+          'git remote set-url origin https://github.com/flutter/flutter.git': null,
           'git describe --tags --abbrev=0': <ProcessResult>[new ProcessResult(0, 0, 'v1.2.3', '')],
         };
         if (platform.isWindows) {
@@ -158,8 +157,7 @@
           'git clone -b dev https://chromium.googlesource.com/external/github.com/flutter/flutter':
               null,
           'git reset --hard $testRef': null,
-          'git remote remove origin': null,
-          'git remote add origin https://github.com/flutter/flutter.git': null,
+          'git remote set-url origin https://github.com/flutter/flutter.git': null,
           'git describe --tags --abbrev=0': <ProcessResult>[new ProcessResult(0, 0, 'v1.2.3', '')],
         };
         if (platform.isWindows) {