Auto merge of #24392 - ferjm:uwp.no.system.gst, r=Manishearth

Allow building and cleaning for UWP without system GStreamer

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

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python/servo/command_base.py

@@ -558,7 +558,9 @@ def get_nightly_binary_path(self, nightly_date):
 
         return self.get_executable(destination_folder)
 
-    def needs_gstreamer_env(self, target, env):
+    def needs_gstreamer_env(self, target, env, uwp=False):
+        if uwp:
+            return False
         try:
             if check_gstreamer_lib():
                 return False
@@ -663,7 +665,7 @@ def build_env(self, hosts_file_path=None, target=None, is_build=False, test_unit
             # Always build harfbuzz from source
             env["HARFBUZZ_SYS_NO_PKG_CONFIG"] = "true"
 
-        if self.needs_gstreamer_env(target or host_triple(), env):
+        if is_build and self.needs_gstreamer_env(target or host_triple(), env, uwp):
             gstpath = gstreamer_root(target or host_triple(), env, self.get_top_dir())
             extra_path += [path.join(gstpath, "bin")]
             libpath = path.join(gstpath, "lib")