Add win-pixel-builder-rel and win-pixel-tester-rel to mb_config.pyl

Test fails
https://ci.chromium.org/p/chromium/builders/ci/win-pixel-builder-rel/49
MBErr: Builder name "win-pixel-builder-rel"  not found under
masters[chromium.fyi]
in "C:\b\s\w\ir\cache\builder\src\tools\mb\mb_config.pyl"

Bug: 965769
Change-Id: I047eb2478ec73c66bc9c1db546931f77b5154e16
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1648748Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Sven Zheng <svenzheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667348}

tools/mb/mb_config.pyl

@@ -297,6 +297,7 @@
       'Win 10 Fast Ring': 'release_trybot',
       'win-annotator-rel': 'release_bot',
       'win-autofill-captured-sites-rel': 'release_bot',
+      'win-pixel-builder-rel': 'release_bot',
       'win32-arm64-rel': 'win32_arm64_release_bot',
     },