[fuchsia] Update fuchsia instruction in Compiling-the-engine.md (#55365)

Behavior of `gclient config --custom-var` got changed, modifying the instruction to edit the raw `.gclient` file to avoid unexpected gclient behavior.

Rich diff: https://github.com/flutter/engine/pull/55365/files?short_path=355a854#diff-355a854159c6f6da4e4e296d0cceb1e7e32ef26af1598aa6cdfaafbdbc656b57
Or rendered md: https://github.com/flutter/engine/blob/b375b805f9bbf478c1618dd0d03a02a11d4e65ab/docs/contributing/Compiling-the-engine.md#compiling-for-fuchsia

Bug: http://b/368770157

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: zijiehe-google-com

docs/contributing/Compiling-the-engine.md

@@ -200,18 +200,28 @@ Also, be sure that Python27 is before any other python in your Path.
 
 ### Build components for Fuchsia
 
-1. Building fuchsia is only supported on linux. You need to run `gclient config --custom-var=download_fuchsia_deps=True` then `gclient sync`.
+1. Building fuchsia is only supported on linux. You need to update `engine/.gclient`, or `../.gclient` if current directory is `engine/src`, with `custom_vars`.
 
-It will set `"download_fuchsia_deps": True` in `"custom_vars"` section in `.gclient` file, and download necessary binaries to build fuchsia components.
+```
+solutions = [
+  {
+    # ...
+    "custom_vars": {
+      "download_fuchsia_deps": True,
+      "run_fuchsia_emu": True,
+    },
+  },
+]
+```
 
-2. If you'd like to run tests locally, also run `gclient config --custom-var=run_fuchsia_emu=True` then `gclient sync`.
+> You may ignore `"run_fuchsia_emu": True` if you won't run tests locally.
 
-It will set `"run_fuchsia_emu": True` in `"custom_vars"` section in `.gclient` file, and download necessary binaries and images to run tests on fuchsia emulators.
-You can set both `custom_vars` and run `gclient sync` only once.
+Run `gclient sync`.
 
-You will also need kvm enabled, or nested virtualization on the gcloud VMs. Fuchsia and the tests will all be executed on the qemu.
+> [!WARNING]
+> When running tests locally, you will also need kvm enabled, or nested virtualization on the gcloud VMs. Fuchsia and the tests will all be executed on the qemu.
 
-3. Prepare and build
+2. Prepare and build
 
 ```
 ./flutter/tools/gn --fuchsia --no-lto
@@ -238,7 +248,7 @@ fuchsia_tests
   * Use `autoninja` if it's available.
   * `-C out/fuchsia_release_x64` for release build; other configurations are similar with a different folder name in `out/`.
 
-4. Run all tests locally
+3. Run all tests locally
 
 ```
 python3 flutter/tools/fuchsia/with_envs.py flutter/testing/fuchsia/run_tests.py
@@ -247,6 +257,10 @@ python3 flutter/tools/fuchsia/with_envs.py flutter/testing/fuchsia/run_tests.py
   * It runs the tests in `out/fuchsia_debug_x64` by default. According to the configuration, it may take 5 minutes with regular gtest output to the terminal.
   * Add `fuchsia_release_x64` at the end of the command for release build; other configurations are similar with a different folder name in `out/`.
 
+```
+python3 flutter/tools/fuchsia/with_envs.py flutter/testing/fuchsia/run_tests.py fuchsia_release_x64
+```
+
 ## Compiling for the Web
 
 For building the engine for the Web we use the [felt](https://github.com/flutter/engine/blob/main/lib/web_ui/README.md) tool.