Clarify the order of operations

Mistakes I made, admittedly low on sleep:
# Used root dispite all warnings
# Ran non-prebuilt code... and couldn't undo.

Clarified by using headings and re-ordering to make prebuilt section stand-alone

BUILD.md

@@ -6,7 +6,7 @@ You may want to build only the client: the server binary, which will be pushed
 to the Android device, does not depend on your system and architecture. In that
 case, use the [prebuilt server] (so you will not need Java or the Android SDK).
 
-[prebuilt server]: #prebuilt-server
+[prebuilt server]: #option-2-prebuilt-server
 
 ## Branches
 
@@ -189,7 +189,18 @@ See [pierlon/scrcpy-docker](https://github.com/pierlon/scrcpy-docker).
 
 ## Common steps
 
-If you want to build the server, install the [Android SDK] (_Android Studio_),
+**As a non-root user**, Clone the project:
+
+```bash
+git clone https://github.com/Genymobile/scrcpy
+cd scrcpy
+```
+
+If you don't want to build the server, use the [Prebuilt server].
+
+### Option 1: If you want to build the server
+
+Install the [Android SDK] (_Android Studio_),
 and set `ANDROID_SDK_ROOT` to its directory. For example:
 
 [Android SDK]: https://developer.android.com/studio/index.html
@@ -203,20 +214,11 @@ export ANDROID_SDK_ROOT=~/Library/Android/sdk
 set ANDROID_SDK_ROOT=%LOCALAPPDATA%\Android\sdk
 ```
 
-If you don't want to build the server, use the [prebuilt server].
-
-Clone the project:
-
-```bash
-git clone https://github.com/Genymobile/scrcpy
-cd scrcpy
-```
-
 Then, build:
 
 ```bash
 meson x --buildtype release --strip -Db_lto=true
-ninja -Cx
+ninja -Cx # DO NOT RUN AS ROOT
 ```
 
 _Note: `ninja` [must][ninja-user] be run as a non-root user (only `ninja
@@ -225,34 +227,7 @@ install` must be run as root)._
 [ninja-user]: https://github.com/Genymobile/scrcpy/commit/4c49b27e9f6be02b8e63b508b60535426bd0291a
 
 
-### Run
+### Option 2: Prebuilt server
-
-To run without installing:
-
-```bash
-./run x [options]
-```
-
-
-### Install
-
-After a successful build, you can install _scrcpy_ on the system:
-
-```bash
-sudo ninja -Cx install    # without sudo on Windows
-```
-
-This installs two files:
-
- - `/usr/local/bin/scrcpy`
- - `/usr/local/share/scrcpy/scrcpy-server`
-
-Just remove them to "uninstall" the application.
-
-You can then [run](README.md#run) _scrcpy_.
-
-
-## Prebuilt server
 
  - [`scrcpy-server-v1.17`][direct-scrcpy-server]  
    _(SHA-256: 11b5ad2d1bc9b9730fb7254a78efd71a8ff46b1938ff468e47a21b653a1b6725_
@@ -271,3 +246,26 @@ sudo ninja -Cx install
 
 The server only works with a matching client version (this server works with the
 `master` branch).
+
+### Run without installing:
+
+```bash
+./run x [options]
+```
+
+### Install
+
+After a successful build, you can install _scrcpy_ on the system:
+
+```bash
+sudo ninja -Cx install    # without sudo on Windows
+```
+
+This installs two files:
+
+ - `/usr/local/bin/scrcpy`
+ - `/usr/local/share/scrcpy/scrcpy-server`
+
+Just remove them to "uninstall" the application.
+
+You can then [run](README.md#run) _scrcpy_.