Auto merge of #21767 - jdm:androidlog, r=paulrouget

Enable use of RUST_LOG with mach run --android.

This allows running `RUST_LOG=layout_thread ./mach run --android` and seeing the appropriate logging output appear in `./adb logcat`.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #21764

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python/servo/post_build_commands.py

@@ -99,6 +99,9 @@ class PostBuildCommands(CommandBase):
             ]
             json_params = shell_quote(json.dumps(params))
             extra = "-e servoargs " + json_params
+            rust_log = env.get("RUST_LOG", None)
+            if rust_log:
+                extra += " -e servolog " + rust_log
             script += [
                 "am start " + extra + " com.mozilla.servo/com.mozilla.servo.MainActivity",
                 "sleep 0.5",