Only check the GStreamer installation when building

Instead of always checking whether GStreamer is installed in mach's
`build_env`, only do this when actually building. Also, use the instance
variable to find features and look for the "media-gstreamer" feature
instead of looking for !"media-dummy."

Fixes #29797.

python/servo/build_commands.py

@@ -113,7 +113,7 @@ class MachCommands(CommandBase):
         if very_verbose:
             opts += ["-vv"]
 
-        env = self.build_env(is_build=True, features=self.features)
+        env = self.build_env(is_build=True)
         self.ensure_bootstrapped()
         self.ensure_clobbered()
 

python/servo/command_base.py

@@ -522,13 +522,14 @@ class CommandBase(object):
             'vcdir': vcinstalldir,
         }
 
-    def build_env(self, hosts_file_path=None, is_build=False, test_unit=False, features=None):
+    def build_env(self, hosts_file_path=None, is_build=False, test_unit=False):
         """Return an extended environment dictionary."""
         env = os.environ.copy()
 
-        if not features or "media-dummy" not in features:
+        if "media-dummy" not in self.features:
             servo.platform.get().set_gstreamer_environment_variables_if_necessary(
-                env, cross_compilation_target=self.cross_compile_target)
+                env, cross_compilation_target=self.cross_compile_target,
+                check_installation=is_build)
 
         if sys.platform == "win32" and type(env['PATH']) == six.text_type:
             # On win32, the virtualenv's activate_this.py script sometimes ends up