[clang] Only build clang in instrumented phase

There's no reason to build everything else, this should save some time in the upload_clang bots.

upload_clang bots pass with this change.

Change-Id: Icb5de7c9fa32ed31dd92651e6a013863ade366a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3641146
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Arthur Eubanks <aeubanks@google.com>
Cr-Commit-Position: refs/heads/main@{#1002404}
NOKEYCHECK=True
GitOrigin-RevId: 244f27c663d55c021c4e9d5aeb1d64cda64e9e31
diff --git a/scripts/build.py b/scripts/build.py
index c35e207..1e4ae2c 100755
--- a/scripts/build.py
+++ b/scripts/build.py
@@ -833,7 +833,7 @@
 
     RunCommand(['cmake'] + instrument_args + [os.path.join(LLVM_DIR, 'llvm')],
                msvc_arch='x64')
-    RunCommand(['ninja'], msvc_arch='x64')
+    RunCommand(['ninja', 'clang'], msvc_arch='x64')
     print('Instrumented compiler built.')
 
     # Train by building some C++ code.